Variants of thermovibrio ammonificans carbonic anhydrase and CO2 capture methods using thermovibrio ammonificans carbonic anhydrase variants

ABSTRACT

The present description relates to recombinant or engineered carbonic anhydrase polypeptides, variants, and functional derivatives thereof, having improved properties that make them advantageous for use in CO 2  capture operations (e.g., CO 2  capture solvents, alkaline pH, and/or elevated temperatures), as well as polynucleotides and vectors encoding same. The present description also relates to methods, processes and systems for CO 2  capture which make use of the recombinant or engineered carbonic anhydrase polypeptides, variants, and functional derivatives thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase Application of PCT International Application No. PCT/CA2016/051049, filed on Sep. 2, 2016, designating the United States of America, which claims priority to U.S. Provisional Application Ser. No. 62/213,941, filed on Sep. 3, 2015, and U.S. Provisional Application Ser. No. 62/323,150, filed on Apr. 15, 2016.

INCORPORATION OF THE SEQUENCE LISTING

The present application contains a Sequence Listing, which is being submitted via EFS-Web on even date herewith. The Sequence Listing is submitted in a file entitled “SeqListing_ROBCA52-005APC.txt,” which was created on Feb. 27, 2018, and is approximately 20 kb in size. This Sequence Listing is hereby incorporated by reference.

TECHNICAL FIELD

The technical field relates to CO₂ capture, Thermovibrio ammonificans carbonic anhydrase (TACA) and/or variants and use thereof for catalyzing the hydration reaction of CO₂ into bicarbonate and hydrogen ions and/or catalyzing the desorption reaction to produce a CO₂ gas.

BACKGROUND

Increasingly dire warnings of the dangers of climate change by the world's scientific community combined with greater public awareness and concern over the issue has prompted increased momentum towards global regulation aimed at reducing man-made greenhouse gas (GHGs) emissions, most notably carbon dioxide. Ultimately, a significant cut in North American and global CO₂ emissions will require reductions from the electricity production sector, the single largest source of CO₂ worldwide. According to the International Energy Agency's (IEA) GHG Program, as of 2006 there were nearly 5,000 fossil fuel power plants worldwide generating nearly 11 billion tons of CO₂, representing nearly 40% of total global anthropogenic CO₂ emissions. Of these emissions from the power generation sector, 61% were from coal fired plants. Although the long-term agenda advocated by governments is replacement of fossil fuel generation by renewables, growing energy demand, combined to the enormous dependence on fossil generation in the near term dictates that this fossil base remain operational. Thus, to implement an effective GHG reduction system will require that the CO₂ emissions generated by this sector be mitigated, with carbon capture and storage (CCS) providing one of the best known solutions.

The CCS process removes CO₂ from a CO₂ containing gas and involves the production of a highly concentrated CO₂ gas stream which is compressed and transported to a geologic sequestration site. This site may be a depleted oil field, a saline aquifer or any suitable storage site. Sequestration in oceans and mineral carbonation are two alternate ways to sequester CO₂ that are in the research phase. Captured CO₂ can also be used for enhanced oil recovery or for carbonation of alkaline waste streams for sequestration as mineral solids.

Conventional technologies for CO₂ capture are based on the use of aqueous amines (e.g. alkanolamines) and carbonates solutions which are circulated through two main distinct units: an absorption unit coupled to a desorption (or stripping) unit. However, in the context of low CO₂ partial pressures encountered in gases from combustion, these conventional technologies give rise to processes with high energy penalty and thus high operational expenditure, as it is the case with monoethanolamine (MEA), or processes with high capital expenditure, as for the case of kinetically limited absorption solutions resulting in large equipment such as with methydiethanolamine (MDEA) for example. Higher pressure CO₂ separation from process streams seen in H₂ production or gasification is typically usually easier to achieve due to the higher pressures in such processes.

Carbonic anhydrase is an enzyme that has been used for CO₂ absorption applications. Carbonic anhydrase is not just a single enzyme form, but a broad group of metalloproteins that exists in genetically unrelated families of isoforms, α, β, γ, δ and ε. Different classes, isoforms and variants of carbonic anhydrase have been used in order to catalyze the hydration reaction of CO₂ into bicarbonate and hydrogen ions and the bicarbonate dehydration reaction into CO₂ and water, as follows: CO₂+H₂O↔H⁺+HCO₃ ⁺  (Reaction 1)

Under optimum conditions, the catalyzed turnover rate of the hydration reaction can reach 1×10⁶ molecules/second.

However, there are several challenges related to the use of carbonic anhydrase in CO₂ capture operations. For instance, the temperature stability in time, the chemical resistance and the activity of the carbonic anhydrase under process conditions are factors that have an impact on process design, process performance and operating costs.

There is thus a need to overcome at least some of the challenges related to the use of carbonic anhydrase for CO₂ capture.

SUMMARY

The present description generally relates to recombinant or engineered carbonic anhydrase (CA) polypeptides, variants, and functional derivatives thereof, having improved properties that make them advantageous for use in CO₂ capture operations. Starting from the wild-type sequence of a carbonic anhydrase from Thermovibrio ammonificans (TACA), directed evolution techniques were employed to identify variants having improved performance in conditions suitable for CO₂-capture (e.g., CO₂ capture solvents, alkaline pH, and/or elevated temperatures). This approached identified a number of amino acid positions, particularly in the N-terminal region of the protein (e.g., residues 1 to 25), which may be engineered to obtain variants having improved properties for CO₂ capture.

Accordingly, the present description relates to the following implementations:

1. A recombinant carbonic anhydrase polypeptide comprising:

-   -   (a) an amino acid sequence having at least 60%, 61%, 62%, 63%,         64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%,         77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%,         90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 99.5%         identity with SEQ ID NO: 2, 4, 6, 7, or 10, or a functional         derivative thereof;     -   (b) an amino acid sequence having at least 60%, 61%, 62%, 63%,         64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%,         77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%,         90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 99.5%         identity with SEQ ID NO: 2, 4, 6, 7, or 10, and comprising at         least one, two, three, four, five, six, seven, eight, nine, ten,         eleven, twelve, thirteen, fourteen, fifteen, sixteen, seventeen,         eighteen, nineteen, twenty, twenty-one, twenty-two,         twenty-three, twenty-four, twenty-five, twenty-six,         twenty-seven, twenty-eight, twenty-nine, thirty, thirty-one,         thirty-two, thirty-three, thirty-four, thirty-five, thirty-six,         thirty-seven, thirty-eight, thirty-nine, forty, forty-one,         forty-two, forty-three, or forty-four amino acid difference(s)         relative to SEQ ID NO: 7 at a position corresponding to X2, X5,         X7, X8, X9, X12, X13, X16, X17, X18, X19, X21, X22, X27, X31,         X50, X51, X52, X73, X76, X77, X82, X101, X103, X115, X116, X117,         X125, X131, X126, X138, X141, X142, X150, X151, X156, X158,         X167, X173, X181, X190, X193, X206, X227, or any combination         thereof;     -   (c) an amino acid sequence having at least 60%, 61%, 62%, 63%,         64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%,         77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%,         90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 99.5%         identity with SEQ ID NO: 2, 4, 6, 7, or 10, and comprising at         least one, two, three, four, five, six, seven, eight, nine, ten,         eleven, twelve, thirteen, fourteen, fifteen, sixteen, seventeen,         eighteen, nineteen, twenty, twenty-one, twenty-two,         twenty-three, twenty-four, twenty-five, twenty-six,         twenty-seven, twenty-eight, twenty-nine, thirty, thirty-one,         thirty-two, thirty-three, thirty-four, thirty-five, thirty-six,         thirty-seven, thirty-eight, thirty-nine, forty, forty-one,         forty-two, forty-three, or forty-four amino acid difference(s)         relative to SEQ ID NO: 7 at a position corresponding to E2X,         W5X, Y7X, S8X, G9X, G12X, P13X, W16X, G17X, D18X, L19X, P21X,         E22X, K27X, N31X, V50X, S51X, V52X, G73X, G76X, Y77X, G82X,         N101X, K103X, D115X, K116X, N117X, V125X, F126X, K131X, K138X,         R141X, V142X, K150X, R151X, R156X, D158X, R167X, S173X, X181X,         F190X, P193X, K206X, Ter227X, or any combination thereof,         wherein C, D, E, F, G, K, L, N, P, Q, R, S, V, W, Y are known         amino acids, Ter is a translation termination codon and X is any         amino acid, or a functional derivative thereof;     -   (d) an amino acid sequence having at least 60%, 61%, 62%, 63%,         64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%,         77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%,         90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 99.5%         identity with SEQ ID NO: 2, 4, 6, 7, or 10, and comprising at         least one, two, three, four, five, six, seven, eight, nine, ten,         eleven, twelve, thirteen, fourteen, fifteen, sixteen, seventeen,         eighteen, nineteen, twenty, twenty-one, twenty-two,         twenty-three, twenty-four, twenty-five, twenty-six,         twenty-seven, twenty-eight, twenty-nine, thirty, thirty-one,         thirty-two, thirty-three, thirty-four, thirty-five, thirty-six,         thirty-seven, thirty-eight, thirty-nine, forty, forty-one,         forty-two, forty-three, or forty-four amino acid difference(s)         relative to SEQ ID NO: 7, wherein said difference comprises X2Q,         X2V, X5C, X5R, X7C, X7F, X7H, X8C, X8R X9C, X9P, X9N, X12D,         X12R, X12V, X13Q, X16C, X16G, X16R, X17C, X18V, X19S, X21L,         X21K, X21D, X21N, X21Q, X21V, X21G, X22K, X22L, X22P, X22N,         X27T, X31C, X50M, X51I, X52I, X73C, X76D, X77D, X82S, X101Q,         X103M, X115N, X116N, X117Q, X125A, X126L, X131I, X138N, X138R,         X141H, X142I, X150M, X151C, X156E, X158G, X167S, X173C, X181Y,         X190C, X190M, X193S, X206T, Ter227YG, or any combination         thereof, wherein Ter is a translation termination codon and A,         C, D, E, G, H, I, K, L, M, N, P, Q, R, S, T, V, Y are known         amino acids and X is any amino acid, or a functional derivative         thereof,     -   (e) an amino acid sequence having at least 60%, 61%, 62%, 63%,         64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%,         77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%,         90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 99.5%         identity with SEQ ID NO: 2, 4, 6, 7, or 10, and comprising at         least one, two, three, four, five, six, seven, eight, nine, ten,         eleven, twelve, thirteen, fourteen, fifteen, sixteen, seventeen,         eighteen, nineteen, twenty, twenty-one, twenty-two,         twenty-three, twenty-four, twenty-five, twenty-six,         twenty-seven, twenty-eight, twenty-nine, thirty, thirty-one,         thirty-two, thirty-three, thirty-four, thirty-five, thirty-six,         thirty-seven, thirty-eight, thirty-nine, forty, forty-one,         forty-two, forty-three, or forty-four amino acid difference(s)         relative to SEQ ID NO: 7, wherein said difference comprises E2Q,         E2V, W5C, W5R, Y7C, Y7F, Y7H, S8C, S8R, G9C, G9P, G9N, G12D,         G12R, G12V, G13Q, W16C, W16G, W16R, G17C, D18V, L19S, P21L,         P21K, P21D, P21N, P21Q, P21V, P21G, E22K, E22L, E22P, E22N,         K27T, N31C, V50M, S51I, V52I, G73C, G76D, Y77D, G82S, N101Q,         K103M, D115N, K116N, N117Q, V125A, F126L, K131I, K138N, K138R,         R141H, V142I, K150M, R151C, R156E, D158G, R167S, S173C, C181Y,         F190C, F190M, P193S, K206T, Ter227YG, or any combination         thereof, wherein A, C, D, E, F, G H, I, K, L, M, N, P, Q, R, S,         T, V, W and Y are known amino acids, Ter is the translation         termination codon, and X is any other amino acid, or a         functional derivative thereof;     -   (f) the amino acid sequence of any one of (a) to (e) comprising         one or more conservative mutations relative to SEQ ID NO: 7; or     -   (g) an amino acid sequence which is at least 60%, 61%, 62%, 63%,         64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%,         77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%,         90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 99.5%         identical to SEQ ID NO: 7, and comprises at least of the amino         acid substitutions: E2Q, E2V, W5C, W5R, Y7C, Y7F, Y7H, S8C, S8R,         G9C, G9P, G9N, G12D, G12R, G12V, G13Q, W16C, W16G, W16R, G17C,         D18V, L19S, P21L, P21K, P21D, P21N, P21Q, P21V, P21G, E22K,         E22L, E22P, E22N, K27T, N31C, V50M, S51I, V52I, G73C, G76D,         Y77D, G82S, N101Q, K103M, D115N, K116N, N117Q, V125A, F126L,         K131I, K138N, K138R, R141H, V142I, K150M, R151C, R156E, D158G,         R167S, S173C, C181Y, F190C, F190M, P193S, K206T, or Ter227YG,         wherein said recombinant polypeptide has carbonic anhydrase         activity.         2. The recombinant polypeptide of implementation 1, wherein the         amino acid sequence of any one of (a) to (f) is different from         SEQ ID NO: 2, 4, 6, 7, or 10.         3. A recombinant carbonic anhydrase polypeptide comprising an         amino acid sequence at least 60%, 61%, 62%, 63%, 64%, 65%, 66%,         67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%,         80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%,         93%, 94%, 95%, 96%, 97%, 98%, 99%, or 99.5% identical to SEQ ID         NO: 7, and a least one amino acid difference relative to SEQ ID         NO: 7 at a position corresponding to residues 1 to 25.         4. The recombinant polypeptide of implementation 3, wherein said         amino acid difference is at a position corresponding to position         2, 5, 7, 8, 9, 12, 13, 16, 17, 18, 19, 21, 22, or any         combination thereof, of SEQ ID NO: 7.         5. The recombinant polypeptide of any one of implementations 1         to 4, which has an improved property relative to the same         property of the polypeptide of SEQ ID NO: 2, 4, 6, 7, or 10,         wherein said improved property is:     -   (a) Improved stability, activity, and/or solubility in presence         of sodium ion;     -   (b) Improved stability, activity, and/or solubility in presence         of potassium ion;     -   (c) Improved stability, activity, and/or solubility in presence         of carbonate ion;     -   (d) Improved stability, activity, and/or solubility under high         pH conditions;     -   (e) Improved stability, activity, and or solubility under high         temperature conditions;     -   (f) Improved stability, activity, and/or solubility under high         ionic strength;     -   (g) Improved pH-activity profile;     -   (h) Improved pH-activity profile under high ionic strength;     -   (i) Improved stability in an amine-based CO₂ capture solvent;     -   (j) Improved stability in an amino-acid-based CO₂ capture         solvent; or     -   (k) Improved stability in a potassium or sodium salt of an         amino-acid based CO₂ capture solvent.         6. A recombinant carbonic anhydrase polypeptide comprising an         amino acid sequence at least 85%, 86%, 87%, 88%, 89%, 90%, 91%,         92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 99.5% identical to         SEQ ID NO: 2, 4, 6, 7 or 10, and having:     -   (a) a cysteine or arginine at a position corresponding to         position 8 of SEQ ID NO: 7;     -   (b) a cysteine, proline, or asparagine at a position         corresponding to position 9 of SEQ ID NO: 7;

(c) a lysine, leucine, proline, or asparagine at a position corresponding to position 22 of SEQ ID NO: 7; or

-   -   (d) any combination of (a) to (c),         wherein said recombinant polypeptide has carbonic anhydrase         activity and exhibits improved stability in a CO₂ capture         solvent as compared to the polypeptide of SEQ ID NO: 2, 4, 6, 7,         or 10.         7. The recombinant polypeptide of implementation 6, wherein said         polypeptide further comprises one or more of the amino acid         substitutions corresponding to E2Q, E2V, W5C, W5R, Y7C, Y7F,         Y7H, S8C, S8R, G9C, G9P, G9N, G12D, G12R, G12V, G13Q, W16C,         W16G, W16R, G17C, D18V, L19S, P21L, P21K, P21D, P21N, P21Q,         P21V, P21G, E22K, E22L, E22P, E22N, K27T, N31C, V50M, S51I,         V52I, G73C, G76D, Y77D, G82S, N101Q, K103M, D115N, K116N, N117Q,         V125A, F126L, K131I, K138N, K138R, R141H, V142I, K150M, R151C,         R156E, D158G, R167S, S173C, C181Y, F190C, F190M, P193S, K206T,         or Ter227YG, of SEQ ID NO: 7.         8. A polynucleotide comprising a nucleotide sequence encoding         the carbonic anhydrase polypeptide as defined in any one of         implementations 1 to 7.         9. The polynucleotide of implementation 8, operably linked to a         heterologous promoter.         10. An expression or cloning vector comprising the         polynucleotide as defined in implementation 8 or 9.         11. A transgenic cell comprising the polynucleotide as defined         in implementation 9, or the expression or cloning vector as         defined in implementation 10.         12. The recombinant carbonic anhydrase polypeptide as defined in         any one of implementations 1 to 7, for use in removing CO₂ from         a CO₂-containing effluent or gas.         13. Use of the carbonic anhydrase polypeptide as defined in any         one of implementations 1 to 7 for removing CO₂ from a         CO₂-containing effluent or gas.         14. A process for treating a CO₂-containing gas, comprising:     -   supplying the CO₂-containing gas to an absorber;     -   supplying an aqueous absorption solution to the absorber;     -   contacting the CO₂-containing gas with the aqueous absorption         solution in the absorber to dissolve the CO₂ into the aqueous         absorption solution, wherein:         -   the aqueous absorption solution comprises a monovalent metal             carbonate compound in a concentration between about 0.1 M             and about 4 M; has a temperature between about 25° C. and             about 80° C.; has an alkaline pH between about 9 and about             11.5 upon entering the absorber; and comprises a carbonic             anhydrase polypeptide as defined in any one of             implementations 1 to 7, free in solution in a concentration             between about 0.05 g/L and about 4 g/L to catalyze the             hydration reaction of the dissolved CO₂ into bicarbonate and             hydrogen ions in the absorber, thereby producing an ion-rich             solution the carbonic anhydrase polypeptide or functional             derivative thereof and a CO₂-depleted gas;     -   removing the ion-rich solution and the CO₂-depleted gas from the         absorber;     -   heating the ion-rich solution to produce a heated ion-rich         solution having a stripping temperature;     -   supplying the heated ion-rich solution to a stripper;     -   converting bicarbonate and hydrogen ions into CO₂ gas and water,         and producing a regenerated ion-depleted solution in the         stripper, wherein:         -   the stripper temperature is higher than the absorber             temperature and is between about 30° C. and about 110° C.;         -   the heated ion-rich solution has a pH in between about 8 and             about 11 upon entering the stripper,         -   the heated ion-rich solution has a CO₂ loading between about             0.05 and about 1 mol CO₂/mol monovalent cation;     -   releasing the CO₂ gas from the stripper;     -   releasing the regenerated ion-depleted solution from the         stripper;     -   cooling at least a portion of the ion-depleted solution to         produce a cooled ion-depleted solution; and     -   recycling at least a portion of the cooled regenerated         ion-depleted solution back to the absorber to form at least part         of the aqueous absorption solution.         15. The process of implementation 14, wherein said         CO₂-containing gas comprises between about 5 vol % and about 50         vol % of CO₂.         16. The process of implementation 14 or 15, wherein said         CO₂-containing gas further comprises CO, SOx, NOx compounds,         NH₃, mercaptans, H₂S, H₂, heavy metals, dusts, ashes, or any         combination thereof.         17. The process of any one of implementations 14 to 16, wherein         said CO₂-containing gas is derived from natural gas combustion,         coal combustion, biogas combustion, biogas upgrading, or natural         gas sweetening.         18. The process of any one of implementations 14 to 17, wherein         the absorber is a packed column.         19. The process of any one of implementations 14 to 18, wherein         the monovalent metal carbonate is potassium carbonate.         20. The process of any one of implementations 14 to 18, wherein         the monovalent metal carbonate is potassium carbonate is in a         concentration between about 0.1 M and about 2 M, or between         about 1.25 M and about 1.75 M.         21. The process of any one of implementations 14 to 20, wherein         the temperature of the aqueous absorption solution in the         absorber is between about 25° C. and about 70° C., preferably         between about 30° C. and about 55° C.         22. The process of any one of implementations 14 to 21, wherein         the pH of the aqueous absorption solution in the absorber is         between about 9 and about 10.5, or between about 9.5 and about         10.5.         23. The process of any one of implementations 14 to 22, wherein         substantially all of the cooled regenerated ion-depleted         solution is recycled back to the absorber to form at least part         of the aqueous absorption solution.         24. The process of any one of implementations 14 to 23, further         comprising adding a make-up carbonic anhydrase polypeptide or         functional derivative thereof component.         25. The process of implementation 24, wherein the make-up         carbonic anhydrase polypeptide or functional derivative thereof         component is added periodically.         26. The process of implementation 24, wherein the make-up         carbonic anhydrase polypeptide or functional derivative thereof         component is added continuously.         27. The process of any one of implementations 24 to 26, wherein         the make-up carbonic anhydrase polypeptide or functional         derivative thereof component comprises an amount of carbonic         anhydrase polypeptide that corresponds to a deactivated amount         of carbonic anhydrase polypeptide cycling between the absorber         and the stripper.         28. The process of implementation 27, further comprising         determining the deactivated amount of carbonic anhydrase         polypeptide or functional derivative thereof, optionally the         determining is done based on sampling and measurements of the         aqueous absorption solution and/or the ion-rich solution, or on         estimates and/or calculations from previously acquired         experimental data, and/or based on overall process performance.         29. The process of any one of implementations 24 to 28, wherein         the make-up carbonic anhydrase polypeptide or functional         derivative thereof component is added into the aqueous         absorption solution prior to entering the absorber.         30. The process of any one of implementations 14 to 29, wherein         the absorber is a packed column or a rotating packed bed (RPB).         31. A method for absorbing CO₂ from a CO₂-containing gas,         comprising:

contacting the CO₂-containing gas with an aqueous absorption solution to dissolve the CO₂ into the aqueous absorption solution at commercial scale process conditions; and

providing the carbonic anhydrase polypeptide as defined in any one of implementations 1 to 7 or functional derivative thereof, to catalyze the hydration reaction of the dissolved CO₂ into bicarbonate and hydrogen ions.

32. The method of implementation 31, further comprising providing operating conditions such that the carbonic anhydrase polypeptide or functional derivative thereof displays enhanced stability and/or activity compared to a reference enzyme.

33. The method of implementation 31 or 32, wherein the carbonic anhydrase polypeptide or functional derivative thereof provides an enhanced CO₂ flux of at least 8.5 times a corresponding CO₂ flux with no enzyme.

34. The method of any one of implementations 31 to 33, wherein the aqueous absorption solution comprises at least one absorption compound.

35. The method of implementation 34, wherein the at least one absorption compound comprises a primary amine, a secondary amine, a tertiary amine, a primary alkanolamine, a secondary alkanolamine, a tertiary alkanolamine, a primary amino acid, a secondary amino acid, a tertiary amino acid, dialkylether of polyalkylene glycols, dialkylether or dimethylether of polyethylene glycol, amino acid or a derivative thereof, monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP), 2-(2-aminoethylamino)ethanol (AEE), 2-amino-2-hydroxymethyl-1,3-propanediol (Tris or AHPD), N-methyldiethanolamine (MDEA), dimethylmonoethanol amine (DMMEA), diethylmonoethanolamine (DEMEA), triisopropanolamine (TIPA), triethanolamine (TEA), DEA, DIPA, MMEA, TIA, TBEE, HEP, AHPD, hindered diamine (HDA), bis-(tertiarybutylaminoethoxy)-ethane (BTEE), ethoxyethoxyethanol-tertiarybutyl amine (EEETB), bis-(tertiarybutylaminoethyl)ether, 1,2-bis-(tertiarybutylaminoethoxy)ethane and/or bis-(2-isopropylaminopropyl)ether, or any combination thereof. 36. The method of implementation 34, wherein the at least one absorption compound comprises a primary amine, a secondary amine, a tertiary amine, a primary alkanolamine, a secondary alkanolamine, a tertiary alkanolamine, a primary amino acid, a secondary amino acid, a tertiary amino acid, or any combination thereof. 37. The method of implementation 34, wherein the at least one absorption compound comprises dialkylether of polyalkylene glycols, dialkylether or dimethylether of polyethylene glycol, amino acid or derivative thereof or a combination thereof. 38. The method of implementation 34, wherein the at least one absorption compound comprises piperazine or derivative thereof, optionally the piperazine or derivatives thereof are substituted by at least one alkanol group. 39. The method of implementation 34, wherein the at least one absorption compound comprises monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP), 2-(2-aminoethylamino)ethanol (AEE), 2-amino-2-hydroxymethyl-1,3-propanediol (Tris or AHPD), N-methyldiethanolamine (MDEA), dimethylmonoethanolamine (DMMEA), diethylmonoethanolamine (DEMEA), triisopropanolamine (TIPA), triethanolamine (TEA), DEA, DTPA, MMEA, TIA, TBEE, HEP, AHPD, hindered diamine (HDA), bis-(tertiarybutylaminoethoxy)-ethane (BTEE), ethoxyethoxyethanol-tertiarybutylamine (EEETB), bis-(tertiarybutylaminoethyl)ether, 1,2-bis-(tertiarybutylaminoethoxy)ethane, and/or bis-(2-isopropylaminopropyl)ether. 40. The method of implementation 34, wherein the at least one absorption compound comprises an amino acid or derivative thereof. 41. The method of implementation 40, wherein the amino acid or derivative thereof comprises glycine, proline, arginine, histidine, lysine, aspartic acid, glutamic acid, methionine, serine, threonine, glutamine, cysteine, asparagine, valine, leucine, isoleucine, alanine, tyrosine, tryptophan, phenylalanine, taurine, N,cyclohexyl 1,3-propanediamine, N-secondary butyl glycine, N-methyl N-secondary butyl glycine, diethylglycine, dimethylglycine, sarcosine, methyl taurine, methyl-α-aminopropionicacid, N-(β-ethoxy)taurine, N-(β-aminoethyl)taurine, N-methyl alanine, 6-aminohexanoic acid, potassium or sodium salt of the amino acid, or any combination thereof. 42. The method of implementation 34, wherein the at least one absorption compound comprises a carbonate compound, preferably sodium carbonate, potassium carbonate or MDEA. 43. The method of implementation 34, wherein the at least one absorption compound comprises sodium carbonate. 44. The method of implementation 34, wherein the at least one absorption compound potassium carbonate. 45. The method of implementation 34, wherein the at least one absorption compound comprises a slow absorption compound. 46. The method of implementation 45, wherein said slow absorption compound comprises tertiary amines, tertiary alkanolamines, tertiary amino-acids, tertiary amino-acid salts, carbonates, or any mixture thereof. 47. The method of any one of implementation 31 to 46, wherein the carbonic anhydrase polypeptide is provided free in the absorption solution at a concentration sufficiently high to cause an increase in foaming of the absorption solution. 48. The method of implementation 47, wherein the absorption solution further comprises a defoamer, an anti-foam compound, or foam-reducer compound (e.g., an oil-in-water emulsion, a polyol based compound, a polyol based dispersion, a silicon based compound, a non-ionic silicon emulsion, and/or a silica particle suspension). 49. The method of implementation 48, wherein the defoamer, an anti-foam compound, or foam-reducer compound is provided in a concentration of at least 10 mg/L, at least 50 mg/L, at least 200 mg/L, between 50 and 300 mg/L, or between 100 and 300 mg/L, based on the volume of the absorption solution. 50. The method of any one of implementations 31 to 49, wherein the temperature of the absorption solution is at least 10° C., preferably at least 25° C. 51. The method of any one of implementations 31 to 50, wherein the step of contacting is performed at a temperature between about 10° C. and about 98° C., preferably between about 25° C. and about 80° C., more preferably between about 30° C. and about 70° C., more preferably between about 40° C. and about 50° C. 52. The method of any one of implementations 31 to 51, wherein the concentration of the carbonic anhydrase polypeptide or functional derivative thereof is between about 0.01 g/L and about 50 g/L in the absorption solution, optionally between about 0.3 g/L and about 10 g/L, optionally between about 0.3 g/L and about 4 g/L. 53. The method of any one of implementations 31 to 52, wherein the pH of the absorption solution is between about 8 and about 11. 54. The method of any one of implementations 31 to 53, wherein the CO₂ loading is between about 0.05 and about 1 mol CO₂/mol amine or mol CO₂/mol cation. 55. The method of any one of implementations 31 to 54, further comprising subjecting the ion-rich solution to desorption to produce a regenerated absorption solution and a CO₂ gas stream. 56. The method of any one of implementations 31 to 55, wherein the carbonic anhydrase polypeptide as defined in any one of implementations 1 to 7 or functional derivative thereof is a component of the absorption solution and the ion-rich solution and catalyzes the desorption reaction. 57. The method of any one of implementations 31 to 56, wherein the absorption is operated at a temperature between about 10° C. and about 98° C., optionally between about 25° C. and about 80° C., between about 30° C. and about 70° C., or between about 40° C. and about 50° C., optionally at 10° C., 20° C., 30° C., 40° C., 50° C., 60° C., 70° C., 80° C., 85° C., 90° C., 95° C. or 98° C. or any other value in between. 58. The method of any one of implementations 31 to 57, wherein the desorption is operated at a temperature between about 30° C. and about 110° C., optionally between about 35° C. and about 90° C. or between about 40° C. and about 70° C. 59. A method for CO₂ capture, comprising:

-   -   in an absorption stage:         -   contacting a CO₂-containing gas with an aqueous absorption             solution to dissolve the CO₂ into the aqueous absorption             solution;     -   providing the carbonic anhydrase polypeptide as defined in any         one of implementations 1 to 7 or functional derivative thereof         in the absorption solution to catalyze the hydration reaction of         the dissolved CO₂ into bicarbonate and hydrogen ions, thereby         producing an ion-rich solution comprising at least some of the         carbonic anhydrase polypeptide or functional derivative thereof         and a CO₂-depleted gas; and/or     -   in a desorption stage:         -   providing conditions for treating the ion-rich solution             comprising at least some of the carbonic anhydrase             polypeptide or the functional derivative thereof, so as to             desorb CO₂ gas from the ion-rich solution, thereby producing             a regenerated absorption solution and a CO₂ gas stream.             60. The method of implementation 59, wherein the absorption             stage is operated with the following absorption operating             parameters:     -   absorption temperature in between about 10° C. and about 98° C.;     -   concentration of an absorption compound in the absorption         solution between about 0.1 M and about 5 M;     -   pH of the absorption solution in between about 8 and about 11;         and/or     -   CO₂ loading in between about 0.05 and about 1 mol CO₂/mol amine         or mol CO₂/mol cation.         61. The method of implementation 59 or 60, wherein the         desorption stage is operated with the following desorption         operating parameter:     -   desorption temperature in between about 30° C. and about 110° C.         62. The method of any one of implementations 59 to 61, wherein         the absorption stage and desorption stage are operated within an         overall operating temperature zone wherein the carbonic         anhydrase polypeptide or functional derivative shows close to         100% residual activity after at least 1 week of exposure to         overall operating temperature zone.         63. The method of any one of implementations 59 to 62, wherein         the absorption stage and desorption stage are operated within an         overall operating temperature zone wherein the carbonic         anhydrase polypeptide or functional derivative provides enhanced         temperature stability compared to a reference enzyme.         64. The method of implementation 63, further comprising one or         more features as defined in any one of implementations 1 to 57.         65. A method for desorption of CO₂ from a solution comprising         bicarbonate and hydrogen ions, comprising providing conditions         desorption of the CO₂ in the presence of the carbonic anhydrase         polypeptide as defined in any one of implementations 1 to 7 or         functional derivative thereof, so as to catalyze the desorption         of CO₂ gas from the solution, thereby producing an ion-depleted         solution and a CO₂ gas stream.         66. The method of implementation 65, further comprising one or         more features as defined in any one of implementations 1 to 64.         67. A system for absorbing CO₂ from a CO₂-containing gas,         comprising:     -   an absorption unit comprising:         -   a gas inlet for receiving the CO₂-containing gas;         -   a liquid inlet for receiving an aqueous absorption solution;         -   a reaction chamber for contacting the CO₂-containing gas             with the aqueous absorption solution to dissolve the CO₂             into the aqueous absorption solution, wherein the carbonic             anhydrase polypeptide as defined in any one of             implementations 1 to 7 or functional derivative thereof is             present for catalyzing the hydration reaction of the             dissolved CO₂ into bicarbonate and hydrogen ions, thereby             producing an ion-rich solution and a CO₂-depleted gas;         -   a liquid outlet for releasing the ion-rich solution; and         -   a gas outlet for releasing the CO₂-depleted gas.             68. The system of implementation 67, further comprising a             regeneration stage for regenerating the ion-rich solution.             69. The system of implementation 68, wherein the             regeneration stage comprises a desorption unit and/or a             mineralization unit             70. The system of any one of implementations 67 to 69,             further comprising a temperature regulator for regulating             the temperature of the absorption unit to promote enhanced             stability of the carbonic anhydrase polypeptide or             functional derivative thereof.             71. The system of any one of implementations 67 to 70,             wherein the operating conditions are provided such that the             combined stability and activity of the carbonic anhydrase             polypeptide or functional derivative thereof provide             enhanced overall CO₂ capture over time per given enzyme             utilization.             72. The system of any one of implementations 67 to 71,             further comprising a make-up device for providing a make-up             carbonic anhydrase polypeptide or functional derivative             thereof to system.             73. The system of implementation 72, wherein the make-up             device comprises a make-up line in fluid communication with             the system.             74. The system of implementation 73, wherein the make-up             line is in fluid communication with the liquid inlet feeding             into the absorption unit for adding the make-up carbonic             anhydrase polypeptide or functional derivative thereof to             the absorption solution.             75. The system of any one of implementations 67 to 74,             further comprising a measurement device configured to             measure a deactivation of the carbonic anhydrase polypeptide             or functional derivative thereof in the system, optionally             the measurement device is configured to retrieve a sample             from the system, determine a sample activity of the carbonic             anhydrase polypeptide or functional derivative thereof in             the sample, compare the sample activity to an initial             activity of the carbonic anhydrase polypeptide, and             determine the deactivation of the carbonic anhydrase             polypeptide or the functional derivative thereof.             76. The system of implementation 75, further comprising a             controller coupled to the measurement device and the make-up             device, the controller being configured to cause the make-up             device to add an amount of the make-up carbonic anhydrase             polypeptide or the functional derivative thereof based on             the deactivation of the carbonic anhydrase polypeptide or             the functional derivative thereof provided by the             measurement device.             77. The system of any one of implementations 72 to 76,             further comprising one or more features of any one of             implementations 1 to 61.             78. An enzyme-enhanced CO₂ capture system, comprising:     -   an absorption unit comprising:         -   a gas inlet for receiving the CO₂-containing gas;         -   a liquid inlet for receiving an aqueous absorption solution;         -   a reaction chamber for contacting the CO₂-containing gas             with the aqueous absorption solution to dissolve the CO₂             into the aqueous absorption solution;         -   the carbonic anhydrase polypeptide as defined in any one of             implementations 1 to 7 or functional derivative thereof that             is present for catalyzing the hydration reaction of the             dissolved CO₂ into bicarbonate and hydrogen ions, thereby             producing an ion-rich solution and a CO₂-depleted gas;         -   a liquid outlet for releasing the ion-rich solution; and         -   a gas outlet for releasing the CO₂-depleted gas;     -   a heat exchanger for heating the ion-rich solution to produce a         heated ion-rich solution;     -   a stripper unit comprising:         -   a liquid inlet for receiving the ion-rich solution;         -   a stripping chamber for allowing CO₂ to be released from the             ion-rich solution to produce CO₂ gas stream and a             regenerated solution, wherein the carbonic anhydrase             polypeptide or functional derivative thereof is present for             catalyzing the dehydration reaction;         -   a liquid outlet for releasing the regenerated solution; and         -   a gas outlet for releasing the CO₂ gas stream; and     -   a recycle system for recycling at least a portion of the         regenerated solution back to the liquid inlet of the absorption         unit as at least part of the aqueous absorption solution.         79. The enzyme-enhanced CO₂ capture system of implementation 78,         further comprising a make-up device for providing make-up         carbonic anhydrase polypeptide or functional derivative thereof         to the system.         80. The enzyme-enhanced CO₂ capture system of implementation 78         or 79, wherein the reaction chamber comprises packing material.         81. The enzyme-enhanced CO₂ capture system of any one of         implementations 78 to 80, wherein the stripping chamber         comprises packing material.         82. The enzyme-enhanced CO₂ capture system of any one of         implementations 78 to 81, wherein the carbonic anhydrase         polypeptide or functional derivative thereof is free in solution         to cyclically flow between the absorption unit and the stripper         unit         83. The enzyme-enhanced CO₂ capture system of any one of         implementations 78 to 82, wherein the carbonic anhydrase         polypeptide or the functional derivative thereof is immobilized         on or in particles that are sized, configured and provided in a         concentration so as to flow with the absorption solution and the         regenerated solution, such that the particles cyclically flow         between the absorption unit and the stripper unit.         84. The enzyme-enhanced CO₂ capture system of any one of         implementations 78 to 83, wherein the CO₂-containing gas is         biogas and/or raw petroleum gas.         85. The enzyme-enhanced CO₂ capture system of any one of         implementations 78 to 84, wherein the CO₂-containing gas is         pretreated by a separated unit to remove contaminants such as         SOx, NOx and H₂S before entering the absorber.         86. A commercial-scale enzyme-enhanced CO₂ capture facility         configured to receive combustion gas comprising CO₂, generated         by a combustion installation, the facility comprising:     -   a feed line for supplying the combustion gas from the combustion         installation;     -   an absorption unit comprising:         -   a gas inlet for receiving the combustion gas;         -   a liquid inlet for receiving an aqueous absorption solution;         -   a reaction chamber for contacting the combustion gas with             the aqueous absorption solution to dissolve the CO₂ into the             aqueous absorption solution;         -   the carbonic anhydrase polypeptide as defined in any one of             implementations 1 to 7 or functional derivative thereof that             is present for catalyzing the hydration reaction of the             dissolved CO₂ into bicarbonate and hydrogen ions, thereby             producing an ion-rich solution and a CO₂-depleted combustion             gas;         -   a liquid outlet for releasing the ion-rich solution; and         -   a gas outlet for releasing the CO₂-depleted combustion gas;     -   a heat exchanger for heating the ion-rich solution to produce a         heated ion-rich solution;     -   a stripper unit comprising:         -   a liquid inlet for receiving the ion-rich solution;         -   a stripping chamber for allowing CO₂ to be released from the             ion-rich solution to produce CO₂ gas stream and a             regenerated solution, wherein carbonic anhydrase polypeptide             or functional derivative thereof is present for catalyzing             the dehydration reaction;         -   a liquid outlet for releasing the regenerated solution; and         -   a gas outlet for releasing the CO₂ gas stream; and     -   a recycle system for recycling at least a portion of the         regenerated solution back to the liquid inlet of the absorption         unit as at least part of the aqueous absorption solution.         87. The facility of implementation 86, wherein the combustion         gas generated by the combustion installation is from coal or         natural gas combustion.         88. The facility of implementation 86 or 87, wherein the feed         line and the absorption unit are configured such that the         combustion gas is supplied to the absorption unit from the         combustion installation without substantial pre-treatment to         remove components from the combustion gas.         89. The facility of any one of implementations 86 to 88, further         comprising one or more features as defined in any one of         implementations 1 to 85.

In some implementations, any one of the methods, systems and/or facilities include one or more features as described above and/or as described in the present description. For instance, the methods, systems and/or facilities may include units, one or more absorption compounds; operating conditions such as temperature, pressure and concentration parameters or using a temperature swing or a pressure swing from absorption to desorption that is between certain temperature ranges described herein; one or more carbonic anhydrase polypeptide or functional derivatives thereof sequences as described herein; different types of CO₂-containing gases to treat; and so on.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the amino acid sequence of wild-type TACA (SEQ ID NO: 2) and its coding nucleic acid sequence (SEQ ID NO: 1). The N-terminal signal peptide (which is eventually cleaved) is underlined and may be replaced with a methionine. DNA sequence was taken from NCBI Reference Sequence: NC_014926.1.

FIG. 2 is a graph of residual activity of various carbonic anhydrases, including TACA (SEQ ID NOs: 4 and 6), after 16 hours incubation in 1.45 M KHCO₃/K₂CO₃ pH 10 (2.9M K⁺) at various temperatures.

FIG. 3 is a graph of residual activity of various carbonic anhydrases, including TACA (SEQ ID NO: 4 and 6), after various incubation times in 1.45 M KHCO₃/K₂CO₃ pH 10 (2.9M K⁺) at 60° C.

FIG. 4 is a graph of residual activity of various carbonic anhydrases, including TACA (SEQ ID NO: 4 and 6), after various incubation times in 1.45 M KHCO₃/K₂CO₃ pH 10 (2.9M K⁺) at 75° C.

FIG. 5 is a graph of residual activity of various carbonic anhydrases, including TACA (SEQ ID NO: 4 and 6), after various incubation times in 1.45 M KHCO₃/K₂CO₃ pH 10 (2.9M K⁺) at 85° C.

FIG. 6 is a graph of residual activity of various carbonic anhydrases, including TACA (SEQ ID NO: 4), after a 1 hour incubation in 1.45 M KHCO₃/K₂CO₃ pH 10 (2.9M K⁺) at 98° C.

FIG. 7 is a graph of residual activity of TACA (SEQ ID NO: 6 or 7) after different thermal cycling times in 1.45 M KHCO₃/K₂CO₃ pH 10 (2.9M K⁺). Temperature profile for one cycle is given in one cycle lasts 8 minutes and is repeated 180 times per day. A total of 28 days was performed, representing a sum of 5040 cycles. Different enzyme concentrations were tested.

FIG. 8 is a graph of residual activity of various carbonic anhydrases, including TACA (SEQ ID NO: 4), after various incubation times in 20% MDEA alpha=0.1 (mol CO₂/mol MDEA) at 60° C.

FIG. 9 is related to thermal cycling described in FIG. 8 and shows temperature fluctuations occurring in one cycle representative of a CO₂ capture process.

FIG. 10 is a process flow diagram illustrating one embodiment, using a CO₂ capture system.

FIG. 11 is another process flow diagram illustrating one embodiment, using a CO₂ capture system including a separation unit.

FIG. 12 shows the residual activity of TACA over time when continuously exposed to a 1.45 M K₂CO₃ pH 10 solution under temperature cycling conditions.

FIG. 13 is a graph showing relative CO₂ absorption rate versus enzyme concentration illustrating the impact of adding TACA to a 1.45 M K₂CO₃ solution at different concentrations on the CO₂ absorption rate of the 1 ton per day CO₂ capture unit.

FIG. 14 is a graph showing residual activity level of TACA (SEQ ID NO: 6 or 7) and one TACA variant containing three mutations (SBR/G9P/E22P) after 1 day incubation at 85° C. in various CO₂ capture solvents.

FIG. 15 presents Table 4 summarizing the different variants identified by the directed evolution screening process. For all shown variants, the stability of a plurality of single mutations (i.e., containing only one amino acid substitution) and combinations of mutations in 1.45 M K₂CO₃ pH 10 at various temperatures, was compared to that of the TACA of SEQ ID NO: 7. Black boxes show variants with a half-life at least 150% that of SEQ ID NO: 7; double-boxes show variants with a half-life at least 125% that of SEQ ID NO: 7; single boxes show variants with a half-life less than 125% that of SEQ ID NO: 7, in at least one of the 3 test conditions of Table 3. Asterix indicates variants not characterized herein as a single mutant.

SEQUENCE LISTING

This description contains a Sequence Listing in computer readable form entitled “Sequence_Listing.txr, created Sep. 1, 2016 having a size of about 20 kb. The computer readable form is incorporated herein by reference,

SEQ ID NO: Description  1 Nucleic acid sequence encoding SEQ ID NO: 2 (FIG. 1)  2 Amino acid sequence of wild-type TACA, including the 20-amino acid N-terminal signal sequence (FIG. 1)  3 Nucleic acid sequence encoding SEQ ID NO: 4  4 Amino acid sequence of wild-type TACA, wherein the 20-amino acid N-terminal signal sequence of the wild- type TACA is replaced with a methionine residue.  5 Nucleic acid sequence encoding SEQ ID NO: 6/7  6/7 Amino acid sequence of an N-terminal truncated wild- type TACA variant displaying higher expression in a bacterial host, wherein the first six amino acids of SEQ ID NO: 4 (“MGGGAH”) are replaced with the four residues “MEHE”, giving the TACA enzyme of SEQ ID NO: 6/7. SEQ ID NOs: 6 and 7 refer to the same protein.  8 Amino acid sequence of carbonic anhydrase from Sulfurihydrogenibium sp. (“SspCA”).  9 Amino acid sequence of a thermostable variant of SspCA referred to as “6M1”. 10 Amino acid sequence of wild-type TACA beginning at the highly conserved tryptophan residue at position 26 of SEQ ID NO: 2, position 7 of SEQ ID NO: 4, or position 5 of SEQ ID NO: 6 or 7.

DETAILED DESCRIPTION

Various methods and/or techniques are provided herein for CO₂ capture using a TACA or functional derivative thereof for catalysis, leveraging the stability and activity of the TACA or functional derivative thereof for operating conditions of the CO₂ capture process.

TACA is a carbonic anhydrase that catalyzes the interconversion of CO₂ and water to bicarbonate and hydrogen ions. TACA is obtained or derived from the thermophilic bacteria Thermovibrio ammonificans (TA) (Giovannelli D, Ricci J, Pérez-Rodriguez I, Hügler M, O'Brien C, Keddis R, Grosche A, Goodwin L, Bruce D, Davenport K W, Dotter C, Han J, Han S, Ivanova N, Land M L, Mikhailova N, Nolan M, Pitluck S, Tapia R, Woyke T, Vetriani C. “Complete genome sequence of Thermovibrio ammonificans HB-1(T), a thermophilic, chemolithoautotrophic bacterium isolated from a deep-sea hydrothermal vent” Standards in Genomic Science 2012 7:82-90.). Methods for isolating/obtaining an enzyme from bacteria are known, such as immunoprecipitation, ultracentrifugation or chromatographic methods. Further details and definitions related to TACA may be found in the Definitions section below. TA grows in the temperature range of 60° C. to 80° C. and optimally at a pH of 5.5.

So far, biochemical study on TACA has been limited. TACA has been the subject of PCT/KR2014/004328. Jo B H, Seo J H, Cha H J, Bacterial extremo-α-carbonic anhydrases from deep-sea hydrothermal vents as potential biocatalysts for CO ₂ sequestration. Journal of Molecular Catalysis B: Enzymatic. 2014, November; 109: p. 31-39 (hereafter “Jo et al.”) and James P, Isupov M N, Sayer C, Saneei V, Berg S, Lioliou M, Kotlar H K, Littlechild J A. The structure of a tetrameric α-carbonic anhydrase from Thermovibrio ammonificans reveals a core formed around intermolecular disulfides that contribute to its thermostability. Acta Crystallogr D Biol Crystallogr. 2014, October; 70 (Pt 10):2607-18 (hereafter “James et al.”), describe preliminary assessment of TACA relative to other known CA enzymes. These works test and assess TACA in relatively mild conditions, such as low-concentrated buffer (pH of about 8) and low ionic strength. However, relatively different process conditions are present in real industrial CO₂ capture applications, which may include conditions such as high pH (e.g., 9 to 11), thermal cycling (temperature swings ranging from 25° C. to 105° C., for example, when cycling from absorption to stripping), very high ionic strength, shear forces, turbulence, and large gas-liquid interfaces which promote mass transfer (yet can have denaturing effects). In addition, due to the relatively high concentrations of carbonate ions contained in various CO₂ capture solvents, proteins can face solubility issues, as reported for example in Yanjie Zhang and Paul S. Cremer. Chemistry of Hofmeister Anions and Osmolytes. Annu Rev Phys Chem. 2010. 61:63-83 (hereafter “Zhang & Cremer”) which describes that the carbonate ion can be a highly efficient protein precipitator.

In addition, neither Jo et al. nor James et al. studied wild type TACA. Jo et al. studied TACA with an extra six histidines tag at the C-terminal end. As shown in the 3D structure of TACA described by James et al., TACA's carboxy terminal functional group is implied in the adoption of a tetrameric organisation. Jo et al. suggest that TACA is a dimeric enzyme while James et al. describe TACA as a tetramer. Moreover, James et al. report that TACA properties can greatly differ according to its oligomerisation state. In James et al., the TACA enzyme which was studied had at its N-terminal end a six histidines tag plus the 20-residues secretion signal. The N-terminal region being close to the active site, significant changes in stability and activity may have occurred.

As will be described further below, signification work, development and testing have been conducted and found that TACA and functional derivatives thereof are operable in the industrial process conditions of a CO₂ capture operation and can provide even greater temperature stability than reported in the literature.

TACA also provides enhanced performance of enzyme-assisted CO₂ capture compared to other CAs, such as Sulfurihydrogenibium sp. (Ssp) CA. Like TA, the bacteria Ssp belongs to the Aquificales order. Ssp was isolated from the Calcite Hot Springs in Yellowstone National Park (USA) and like TA, grows in 60° C. to 80° C. temperature range. Sulfurihydrogenibium yellowsfonense sp. nov. is an extremely thermophilic, facultatively heterotrophic, sulfur-oxidizing bacterium from Yellowstone National Park. This bacteria along with Sulfurihydrogenibium subterraneum and Sulfurihydrogenibium azorense are described in Nakagawa S, Shtaih Z, Banta A, Beveridge T J, Sako Y, Reysenbach A L. International Journal of Systematic and Evolutionary Microbiology, 2005 November; 55(Pt 6):2263-8. (PubMed ID 16280480).

Distinctly, Ssp grows optimally at pH 7.5, a value two orders of magnitude higher than that of TA. Ssp genome contains a gene encoding for an alpha-class carbonic anhydrase hereafter referred as SspCA. Some recent biochemical characterizations of SspCA are reported in the literature. However, it is hard to expect or predict TACA properties based on those of SspCA, given that the two proteins share only 49% sequence identity.

Both SspCA and TACA are believed to be secreted after being produced because of the presence of a signal peptide. In that context, TACA and SspCA have to deal with conditions occurring outside the bacteria. Because of the different optimal growth pH of Ssp vs TA, one could expect SspCA to be more robust than TACA when dissolved in CO₂ capture solvents, the latter being alkaline with pH ranging from 8 to 11. However, embodiments of the present description provide results revealing that TACA stability is surprisingly much higher than that of SspCA in tested relevant CO₂ capture solvents and conditions.

Referring to FIG. 1, an amino acid sequence of a TACA is illustrated. The cleaved signal peptide is underscored and may be replaced with a methionine (SEQ ID NO: 4). Various TACA variants and functional derivatives may also be used in the CO₂ capture techniques described herein. For example, the first six amino acids of the TACA of SEQ ID NO: 4 (“MGGGAH”) were replaced by four other amino acids (“MEHE”), giving the TACA of SEQ ID NO: 6/7. This change was performed in order to increase enzyme production level and did not have a measureable negative impact on TACA stability (FIGS. 2 to 5). Table 3 describes many other stabilized variants from SEQ ID NO: 7 that were constructed using directed evolution approaches.

Referring now to FIG. 10, an example of the overall CO₂ capture system 10 includes a source 12 of CO₂ containing gas 14. The source may be a power plant, an aluminum smelter, refinery or another type of CO₂ producing operation at high or atmospheric pressure, or may also be ambient air for some specific applications such as air fractionation, air cleaning, or biogas. The CO₂ containing gas 14 is supplied to an absorption unit 16, which is also fed with an aqueous absorption solution 18 for contacting the CO₂ containing gas 14. In some implementations, the aqueous absorption solution 18 includes a carbonic anhydrase which is TACA or a functional derivative thereof and an absorption compound. The carbonic anhydrase may be free in the aqueous absorption solution 18 as dissolved enzyme or aggregates or particles of enzymes. The carbonic anhydrase may be on or in particles that are present in the aqueous absorption solution 18 and flow with it through the absorption unit 16. The carbonic anhydrase may be immobilized with respect to the particles using any method while keeping at least some of its activity. Some immobilization techniques include covalent bonding, entrapment, encapsulation, and so on. The carbonic anhydrase may be immobilized with respect to supports, which may be various structures such as packing material, within the absorption unit 16 so as to remain within the absorption unit 16 as the aqueous absorption solution 18 flows through it.

The CO₂ containing gas 14 may be a CO₂-containing effluent from various sources that includes a proportion of CO₂ and other gases. For example the gas may include from about 0.03% to 60% (v/v) of CO₂ although the CO₂ concentration may be greater. The CO₂-containing gas may also be a gas having high CO₂ content up to 100%, which may be useful for the production of compounds such as sodium bicarbonate from CO₂ gas as one of the starting materials.

The absorption unit 16 (also referred to as an “absorber” herein) may be of various types, such as a packed reactor, a spray reactor, a bubble column type reactor, a rotating packed bed (RPB) or other type of process intensification (PI) reactor, and so on. There may be one or more reactors that may be provided in series or in parallel. In the absorption unit 16, the TACA or functional derivative thereof catalyses the hydration reaction of CO₂ into bicarbonate and hydrogen ions and thus a CO₂ depleted gas 20 and an ion-rich solution 22 are produced.

The ion-rich solution 22 is then supplied to a desorption unit 26 (also referred to herein as a “stripper”) to produce a CO₂ stream 28 and an ion depleted solution 30. The TACA or functional derivative thereof may also be present to catalyse the dehydration reaction of bicarbonate ions into CO₂ and thus a CO₂ depleted gas 20 and an ion depleted solution 30 is produced. Alternatively, the ion-rich solution 22 may be supplied to another type of regeneration step such as mineral carbonation and the like. It should be noted that the ion-rich solution 22 may be heated prior to being supplied to the desorption unit 26.

Referring now to FIG. 11, the system 10 may also include a separation unit 32 arranged in between the absorption unit 16 and the desorption unit 26, for removing at least some and possibly all of the TACA or functional derivative thereof in the event the enzyme is flowing with the ion-rich solution 22, e.g. when the enzyme is free in solution or immobilized with respect to flowing particles. The separation unit 32 produces an enzyme depleted stream 34 that may be supplied to the desorption unit 26 and an enzyme rich stream 36 that may be recycled, in whole or in part, to the absorption unit 16. The separation unit may also include one or more separators in series or parallel. The separators may be filters or other types of separators, depending on the removal characteristics for the enzymes and the form of the enzymes or particles.

The system may also include various other treatment units for preparing the ion-rich solution 22 for the desorption unit 26 and/or for preparing the ion depleted solution 30 for recycling into the absorption unit 16. There may be pH adjustment units or various monitoring units.

In some implementations, at least one TACA or functional derivative thereof is provided in the desorption unit 26. The TACA or functional derivative thereof may be provided within the input ion-rich solution and/or added separately. The TACA or functional derivative thereof may be tailored, designed, immobilised or otherwise delivered in order to withstand the conditions in the desorption unit 26. The TACA or the functional derivative thereof may catalyze the conversion of bicarbonate ion to CO₂ as described in Reaction 1 (reverse reaction).

Referring still to FIG. 11, the system may also include a measurement device 40 for monitoring properties of various streams and adjusting operation of the absorption unit 16 to achieve desired properties. Adjusting could be done by various methods including modifying the liquid and/or gas flow rates, for example, or adjusting other operating conditions. In some implementations, the measurement device 40 can monitor the activity of the TACA or the functional derivative thereof cycled through the CO₂ capture system 10, and this information can be used to determine, calibrate and/or control the addition of the make-up TACA or functional derivative thereof into the system.

In some implementations, the absorption unit 16 may be operated at conditions so as to leverage the activity and/or stability of the TACA or functional derivative thereof used to catalyze the CO₂ hydration reaction. For example, it has been found that TACA or the functional derivative thereof can present high residual activity over a range of elevated temperatures in aqueous absorption solutions including sodium carbonate or potassium carbonate. The TACA or functional derivative thereof also presents high activity at lower ambient temperature to provide elevated CO₂ flux in aqueous absorption solutions including sodium carbonate, potassium carbonate or alkanolamines such as MDEA. The operating conditions may include an operating temperature and at least one operating absorption compound within the absorption solution. The operating conditions may further include pH, CO₂ loading, gas and liquid flow rates and compositions, and so on.

In some implementations, the operating conditions are coordinated for maximum leverage of the TACA or functional derivative thereof functionality in CO₂ capture. In some implementations, the operating conditions are provided for commercial scale CO₂ capture operations—such as relatively high pH, high ionic strength, high temperature, and so on—and the TACA or functional derivative or variant thereof provides high performance for catalysis of the desired reaction(s) in the cyclic system.

In some implementations, the operating conditions may include temperature conditions that, depending on various other parameters of the CO₂ capture operation, may provide an absorption temperature higher than 10° C. and lower than 98° C., such as between 25 and 80° C., 30 and 70° C. or 30 and 50° C. or such as 15° C., 20° C., 25° C., 30° C., 35° C., 40° C., 45° C., 50° C., 55° C., 60° C., 65° C., 70° C., 75° C., 80° C., 85° C., 90° C., 95° C., 98° C., or any temperature in between. It should also be understood that the temperature conditions in the absorption unit may vary within a certain temperature range, since the operating temperatures at different locations within the absorption unit will be different. In addition, the temperature of the absorption solution can substantially fluctuate throughout absorption and desorption stages that can be used in some CO₂ capture operations.

In some implementations, the operating conditions may include pressure conditions that, depending on various other parameters of the CO₂ capture operation, may provide an absorption pressure higher than 1 bar and lower than 100 bar, such as 2 bars, 5 bars, 10 bars, 20 bars, 25 bars, 30 bars, 35 bars, 40 bars, 45 bars, 50 bars, 55 bars, 60 bars, 65 bars, 70 bars, 75 bars, 80 bars, 85 bars, 90 bars, 95 bars, 100 bars, or any pressure in between.

In some implementations, the operating conditions may include temperature conditions that, depending on various other parameters of the CO₂ capture operation, may provide a desorption temperature higher than 10° C. and lower than 110° C., such as between 30 and 110° C., 35 and 90° C. or 40 and 70° C. or such as 15° C., 20° C., 25° C., 30° C., 35° C., 40° C., 45° C., 50° C., 55° C., 60° C., 65° C., 70° C., 75° C., 80° C., 85° C., 90° C., 95° C., 100° C., 105° C., 110° C. or any temperature in between. It should also be understood that the temperature conditions in the desorption unit may vary within a certain temperature range, since the operating temperatures at different locations within the desorption unit will be different. In addition, the temperature of the absorption solution can substantially fluctuate throughout absorption and desorption stages that can be used in some CO₂ capture operations. It should also be noted that the operating conditions may include a temperature swing between the absorption unit and the desorption unit, and the temperature swing may vary between about 25° C. and about 105° C., optionally between about 30° C. and about 85° C., or between about 40° C. and about 60° C., for example. Different temperature swings can be used depending on various operating parameters, such as type of solvent or absorption compound(s) used in the process.

In some implementations, the operating conditions may include pressure conditions that, depending on various other parameters of the CO₂ capture operation, may provide a desorption absolute pressure higher than 0.05 bar and lower than 50 bars, such as 0.1 bar, 0.2 bars, 0.3 bar, 0.4 bar, 0.5 bar, 0.6 bar, 0.7 bar, 0.8 bar, 0.9 bar, 1 bar, 2 bars, 5 bars, 10 bars, 15 bars, 20 bars, 25 bars, 30 bars, 35 bars, 40 bars, 45 bars, 50 bars or any absolute pressure in between.

In some implementations, the operating conditions may include an aqueous absorption solution including an absorption compound, which will be further discussed below.

The enzyme is preferably used in combination with an absorption solution that will supply the CO₂ carrying capacity for the process. The solution may have a composition allowing acceleration of the enzyme catalytic rate by capturing the hydrogen ion released during the hydration reaction. Using TACA or a functional derivative thereof allows the CO₂ capture operation to be accelerated, reducing the size of the required capture vessels and associated capital costs. In addition, by taking advantage of this accelerative mechanism, energetically favorable absorption compounds such as tertiary and hindered amines, carbonate/bicarbonate solutions and amino acids/amino acid salts can be employed to reduce associated process energy consumption, where these absorption compounds would normally be too slow to be used efficiently without enzymatic catalysis.

The aqueous absorption solution may include at least one absorption compound that aids in the absorption of CO₂. The absorption compound may include potassium carbonate, sodium carbonate, ammonium carbonate, and/or at least one amine, which may be a primary amine, a secondary amine, a tertiary amine, a primary alkanolamine, a secondary alkanolamine, a tertiary alkanolamine, and/or an amino acid with primary, secondary or tertiary amino group(s) or a combination thereof. Combinations of absorption compounds include a carbonate and at least one of the amines and/or amino acids mentioned therein or herein, to produce a promoted carbonate absorption solution. It should also be noted that the absorption solution can include a single absorption compound, such as potassium carbonate. In addition, the absorption solution can include a main absorption compound, such as potassium carbonate, and also one or more secondary compounds that may include an amine, for example.

In some scenarios, the absorption compound may include monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP), 2-(2-aminoethylamino)ethanol (AEE), 2-amino-2-hydroxymethyl-1,3-propanediol (Tris or AHPD), N-methyldiethanolamine (MDEA), dimethylmonoethanolamine (DMMEA), diethylmonoethanolamine (DEMEA), triisopropanolamine (TIPA), triethanolamine (TEA), DEA, DIPA, MMEA, TIA, TBEE, HEP, AHPD, hindered diamine (HDA), bis-(tertiarybutylaminoethoxy)-ethane (BTEE), ethoxyethoxyethanol-tertiarybutylamine (EEETB), bis-(tertiarybutylaminoethyl)ether, 1,2-bis-(tertiarybutylaminoethoxy)ethane and/or bis-(2-isopropylaminopropyl)ether, and the like.

In some scenarios, the absorption compound may include piperidine, piperazine, derivatives of piperidine, piperazine which are substituted by at least one alkanol group, dialkylether of polyalkylene glycols, dialkylether or dimethylether of polyethylene glycol, amino acids comprising glycine, proline, arginine, histidine, lysine, aspartic acid, glutamic acid, methionine, serine, threonine, glutamine, cysteine, asparagine, valine, leucine, isoleucine, alanine, tyrosine, tryptophan, phenylalanine, and derivatives such as taurine, N,cyclohexyl 1,3-propanediamine, N-secondary butyl glycine, N-methyl N-secondary butyl glycine, diethylglycine, dimethylglycine, sarcosine, methyl taurine, methyl-α-aminopropionic acid, N-(β-ethoxy)taurine, N-(β-aminoethyl)taurine, N-methyl alanine, 6-aminohexanoic acid, potassium or sodium salt of the amino acid or a combination thereof.

The absorption compound used to make up the aqueous absorption solution may be at least one of the example compounds, i.e. potassium carbonate, sodium carbonate and/or MDEA.

In some scenarios, the concentration of the absorption compound in the solution may be between about 0.1 M and about 10 M, depending on various factors. When the absorption compound is amine-based, the concentration of the amine-based solution may be between about 0.1 M and 8 M and when the absorption compound is amino acid-based, the concentration of the amino acid-based solution may be between about 0.1 M and 6 M. When the absorption compound is carbonate based, the pH of the absorption solution may be between about 8 and about 12, depending for example on the absorption compound and on the CO₂ loading of the solution.

The TACA or functional derivative thereof may be dissolved in the absorption solution. The concentration of the TACA or functional derivative thereof may be between about 0.01 and about 50 g/L, between about 0.01 and about 10 g/L or between about 0.1 and about 5 g/L. When the TACA or functional derivative thereof is not dissolved in the solution but is rather immobilized on mobile particles or fixed on packing material, the amount of immobilized TACA or functional derivative thereof may be similar so as to provide a similar activity as the therein mentioned concentrations of dissolved TACA or functional derivative thereof.

As noted above, the TACA or functional derivative thereof may be provided free or dissolved in the solvent, immobilized or entrapped or otherwise attached to particles that are in the absorption solution or to packing material or other structures that are fixed within the reaction chamber.

In the case where the TACA or functional derivative thereof is immobilized with respect to a support material, this may be accomplished by an immobilization technique selected from adsorption, covalent bonding, entrapment, copolymerization, cross-linking, and encapsulation, or combination thereof.

In one scenario, the TACA or functional derivative thereof may be immobilized on a support that is in the form of particles, beads or packing. Such supports may be solid or porous with or without coating(s) on their surface. The TACA or functional derivative thereof may be covalently attached to the support and/or the coating of the support, or entrapped inside the support or the coating. The coating may be a porous material that entraps the TACA or functional derivative thereof within pores and/or immobilizes the TACA by covalent bonding to the surfaces of the support. The support material may be made from a compound different than the TACA or functional derivative thereof. The support material may include nylon, cellulose, silica, silica gel, chitosan, polyacrylamide, polyurethane, alginate, polystyrene, polymethylmetacrylate, magnetic material, sepharose, titanium dioxide, zirconium dioxide and/or alumina, respective derivatives thereof, and/or other materials. The support material may have a density between about 0.6 g/ml and about 5 g/ml such as a density above 1 g/ml, a density above 2 g/mL, a density above 3 g/mL or a density of about 4 g/mL.

In some scenarios, the TACA or functional derivative thereof may be provided as cross-linked enzyme aggregates (CLEAs) and/or as cross-linked enzyme crystals (CLECs).

In the case of using enzymatic TACA or functional derivative thereof particles, including CLEAs or CLECs, the particles may be sized to have a diameter at or below about 17 μm, optionally about 10 μm, about 5 μm, about 4 μm, about 3 μm, about 2 μm, about 1 μm, about 0.9 μm, about 0.8 μm, about 0.7 μm, about 0.6 μm, about 0.5 μm, about 0.4 μm, about 0.3 μm, about 0.2 μm, about 0.1 μm, about 0.05 μm, or about 0.025 μm. The particles may also have a distribution of different sizes.

The TACA or functional derivative thereof used in connection with the techniques described herein may be an isolated and/or substantially pure form.

There is also provided a carbonic anhydrase polypeptide or functional derivatives thereof, which is stable and active at a broad range of temperatures.

In one implementation, the carbonic anhydrase is a polypeptide comprising the sequence as set forth in SEQ ID NO: 2, 4, 6 or 7 or functional derivative thereof, an expression or cloning vector comprising a nucleotide sequence encoding such carbonic anhydrase, and a transgenic cell comprising such expression or cloning vector.

The TACA or the derivative thereof can be used in various processes and scenarios such as those described in the following patent references that are hereby incorporated herein by reference: CA 2,291,785; CA 2,329,113; CA 2,393,016; CA 2,443,222; U.S. Pat. No. 6,908,507; EP 1 377 531, U.S. Pat. Nos. 7,514,056; 7,596,952; 8,066,965; 8,277,769; 6,946,288; 7,740,689; WO2012/103653; US 2013/0203155; CA 2,769,771; US 2012/0122195; U.S. Pat. No. 8,722,391; CA 2,554,395; CA 2,738,061; WO2014/066999; CA 2,886,708.

DEFINITIONS

In order to further appreciate some of the terms used herein, the following definitions and discussion are provided.

The term “a” or “the” refers to “at least one” and can cover “several”.

The expression “polypeptide” refers to any peptide or protein comprising two or more amino acids joined to each other by peptide bonds or modified peptide bonds. “Polypeptide(s)” refers to both short chains, commonly referred to as peptides, oligopeptides and oligomers, and to longer chains generally referred to as proteins. Polypeptides may contain amino acids other than the 20 gene-encoded amino acids, optionally polypeptides may contain glycine, proline, arginine, histidine, lysine, aspartic acid, glutamic acid, methionine, serine, threonine, glutamine, cysteine, asparagine, valine, leucine, isoleucine, alanine, tyrosine, tryptophan, phenylalanine, selenocysteine, selenomethionine, pyrrolysine. “Polypeptide(s)” include those modified either by natural processes, such as processing and other post-translational modifications, but also by chemical modification techniques. Such modifications are well described in basic texts and in more detailed monographs, as well as in a voluminous research literature and they are well known to those of skill in the art. It will be appreciated that the same type of modification may be present in the same or varying degree at several sites in a given polypeptide.

The expression “functional derivative” refers to a protein/peptide/polypeptide sequence that possesses a functional biological activity that is substantially similar to the biological activity of the original protein/peptide/polypeptide sequence. In other words, it refers to a polypeptide of the carbonic anhydrase as defined herein that substantially retain(s) the capacity of catalyzing the hydration of carbon dioxide. In this description, the term “TACA” includes its functional derivatives, which have carbonic anhydrase activity. A functional derivative of the carbonic anhydrase protein/peptide as defined herein may or may not contain post-translational modifications such as covalently linked carbohydrates, if such modifications are not necessary for the performance of a specific function. The “functional derivative” may also comprise nucleic acid sequence variants encoding the protein/peptide/polypeptide of the present description. These variants may result from the degeneracy of the genetic code or from a mutation, substitution, addition or deletion. Further, the carbonic anhydrase as defined herein may comprise a Tag such as a histidine Tag. The term “functional derivative” is meant to encompass the “variants”, the “mutants”, the “fragments” or the “chemical derivatives” of a carbonic anhydrase protein/peptide. Methods for measuring carbonic anhydrase activity are known such as stirred cell reactor assay or the method described by Chirica et al. (Chirica et al. European Journal of Biochemistry, 1997, 244, 755-60). These functional derivatives have at least 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% 99% or 99.5% identity with the sequence as set forth in SEQ ID NO: 2, 4, 6, 7, or 10 optionally over the entire length of the sequence or on a partial alignment of the sequences.

The term “polynucleotide fragment”, as used herein, refers to a polynucleotide whose sequence (e.g., cDNA) is an isolated portion of the subject nucleic acid constructed artificially (e.g., by chemical synthesis) or by cleaving a natural product into multiple pieces, using restriction endonucleases or mechanical shearing, or a portion of a nucleic acid synthesized by PCR, DNA polymerase or any other polymerizing technique well known in the art, or expressed in a host cell by recombinant nucleic acid technology well known to one of skill in the art.

The term “polypeptide or fragments thereof” as used herein refers to peptides, oligopeptides and proteins. This term also does not exclude post-expression modification of polypeptides. For example, polypeptides that include the covalent attachment of glycosyl groups, acetyl groups, lipid groups and the like are encompassed by the term polypeptide.

Techniques for determining nucleic acid and amino acid “sequence identity” are known in the art. Typically, such techniques include determining the nucleotide sequence of the mRNA for a gene and/or determining the amino acid sequence encoded thereby, and comparing these sequences to a second nucleotide or amino acid sequence. In general, “identity” refers to an exact nucleotide-to-nucleotide or amino acid-to-amino acid correspondence of two polynucleotides or polypeptide sequences, respectively. Two or more sequences (polynucleotide or amino acid) can be compared by determining their “percent identity.” The percent identity of two sequences, whether nucleic acid or amino acid sequences, is the number of exact matches between two aligned sequences divided by the length of the shorter sequence and multiplied by 100. An approximate alignment for nucleic acid sequences is provided by the local homology algorithm of Smith and Waterman, Advances in Applied Mathematics 2:482-489 (1981). This algorithm can be applied to amino acid sequences by using the scoring matrix developed by Dayhoff, Atlas of Protein Sequences and Structure, M. O. Dayhoff ed., 5 suppl. 3:353-358, National Biomedical Research Foundation, Washington, D.C., USA, and normalized by Gribskov, Nucl. Acids Res. 14(6):6745-6763 (1986). An exemplary implementation of this algorithm to determine percent identity of a sequence is provided by the Genetics Computer Group (Madison, Wis.) in the “BestFit” utility application. The default parameters for this method are described in the Wisconsin Sequence Analysis Package Program Manual, Version 8 (1995) (available from Genetics Computer Group, Madison, Wis.). Another method of establishing percent identity which can be used in the context of the present description is the MPSRCH package of programs copyrighted by the University of Edinburgh, developed by John F. Collins and Shane S. Sturrok, and distributed by IntelliGenetics, Inc. (Mountain View, Calif.). From this suite of packages the Smith-Waterman algorithm can be employed where default parameters are used for the scoring table (for example, gap open penalty of 12, gap extension penalty of one, and a gap of six). From the data generated the “Match” value reflects “sequence identity.” Other suitable programs for calculating the percent identity between sequences are generally known in the art, for example, another alignment program is BLAST, used with default parameters. For example, BLASTN and BLASTP can be used using the following default parameters: genetic code=standard; filter=none; strand=both; cutoff=60; expect=10; Matrix BLOSUM62; Descriptions=50 sequences; sort by=HIGH SCORE; Databases=non-redundant, GenBank+EMBL+DDBJ+PDB+GenBank CDS translations+Swiss protein+Spupdate+PIR.

As used herein, the phrase “at a position corresponding to the position of and similar phrases, refer to the fact that a person of skill in the art would be able to perform for example a multiple sequence alignment of one or more proteins of a given family (e.g., carbonic anhydrases) to determine whether two amino acids in two different proteins of different lengths and/or residue numbering correspond to the same position within the secondary or tertiary structure of the protein.

By “substantially identical” when referring to a polypeptide, it will be understood that the polypeptide of the present description preferably has an amino acid sequence having at least about 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 99.5% or any other value in between to SEQ ID NO: 2, 4, 6, 7 or 10, or functional derivatives thereof, optionally over the entire length of the peptide.

One can use a program such as the CLUSTAL program to compare amino acid sequences. This program compares amino acid sequences and finds the optimal alignment by inserting spaces in either sequence as appropriate. It is possible to calculate amino acid identity or homology for an optimal alignment. A program like BLASTp will align the longest stretch of similar sequences and assign a value to the fit. It is thus possible to obtain a comparison where several regions of similarity are found, each having a different score. Both types of identity analysis are contemplated for the present description.

With respect to protein or polypeptide, the term “isolated polypeptide” or “isolated and purified polypeptide” is sometimes used herein. This term refers primarily to a protein produced by expression of an isolated and modified polynucleotide molecule contemplated by the present description. Alternatively, this term may refer to a protein which has been sufficiently separated from other proteins with which it would naturally be associated, so as to exist in “substantially pure” form.

The term “substantially pure” refers to a preparation comprising at least 50% by weight of the carbonic anhydrase polypeptide or derivative thereof on total protein content. More preferably, the preparation comprises at least 75% by weight, and most preferably 90-99% by weight, of the carbonic anhydrase polypeptide or derivative thereof.

Purity is measured by methods appropriate for the carbonic anhydrase polypeptide or derivative thereof as described herein (e.g. chromatographic methods, agarose or polyacrylamide gel electrophoresis, HPLC analysis, and the like).

The TACA polypeptide or TACA functional derivative thereof can also comprise amino acids substitution such that the carbonic anhydrase or TACA functional derivative thereof retains catalytic activity (i.e. the interconversion of CO₂ with HCO₃ and H⁺). The term “substituted amino acid” is intended to include natural amino acids and non-natural amino acids. Non-natural amino acids include amino acid derivatives, analogues and mimetics. As used herein, a “derivative” of an amino acid refers to a form of the amino acid in which one or more reactive groups on the compound have been derivatized with a substituent group. As used herein an “analogue” of an amino acid refers to a compound that retains chemical structures of the amino acid necessary for functional activity of the amino acid yet also contains certain chemical structures that differ from the amino acid. As used herein, a “mimetic” of an amino acid refers to a compound that mimics the chemical conformation of the amino acid.

As used herein, the term “polynucleotide(s)” generally refers to any polyribonucleotide or poly-deoxyribonucleotide, which may be unmodified RNA or DNA or modified RNA or DNA. This definition includes, without limitation, single- and double-stranded DNA, DNA that is a mixture of single- and double-stranded regions or single-, double- and triple-stranded regions, cDNA, single- and double-stranded RNA, and RNA that is a mixture of single- and double-stranded regions, hybrid molecules comprising DNA and RNA that may be single-stranded or, more typically, double-stranded, or triple-stranded regions, or a mixture of single- and double-stranded regions. The term “polynucleotide(s)” also embraces short nucleotides or fragments, often referred to as “oligonucleotides”, that due to mutagenesis are not 100% identical but nevertheless code for the same amino acid sequence.

By “substantially identical” when referring to a polynucleotide, it will be understood that the polynucleotide of the present description has a nucleic acid sequence which encodes a polypeptide which is at least about 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 99.5% or any other value between 60 and 99.5% identical to SEQ ID NO: 2, 4 or 6 or functional derivative thereof.

By “substantially identical” when referring to a polynucleotide, it will be understood that the polynucleotide of the present description has a nucleic acid sequence which is at least about 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 99.5% or any other value between 60 and 99.5% identical to SEQ ID NO: 1, 3 or 5 or functional derivative thereof.

With reference to polynucleotides described herein, the term “isolated polynucleotide” is sometimes used. This term, when applied to DNA, refers to a DNA molecule that is separated from sequences with which it is immediately contiguous to (in the 5′ and 3′ directions) in the naturally occurring genome of the organism from which it was derived. For example, the “isolated polynucleotide” may comprise a DNA molecule inserted into a vector, such as a plasmid or virus vector, or integrated into the genomic DNA of a prokaryote or eukaryote. An “isolated polynucleotide molecule” may also comprise a cDNA molecule.

As used herein, the term “vector” refers to a polynucleotide construct designed for transduction/transfection of one or more cell types. Vectors may be, for example, cloning vectors which are designed for isolation, propagation and replication of inserted nucleotides, expression vectors which are designed for transcription of a nucleotide sequence in a host cell, or a viral vector which is designed to result in the production of a recombinant virus or virus-like particle, or shuttle vectors, which comprise the attributes of more than one type of vector. A number of vectors suitable for stable transfection of cells and bacteria are available to the public (e.g. plasmids, adenoviruses, baculoviruses, yeast baculoviruses, plant viruses, adeno-associated viruses, retroviruses, Herpes Simplex Viruses, Alphaviruses, Lentiviruses), as are methods for constructing such cell lines. It will be understood that the present application encompasses any type of vector comprising any of the polynucleotide molecules of the present description.

The term “transgenic cell” refers to a genetically engineered cell. Methods for genetically engineering a cell are known such as molecular cloning and gene targeting. These methods can include chemical-based transfection, non-chemical method, particle-based method or viral method. The host cell may be any type of cell such as a transiently-transfected or stably-transfected mammalian cell line, an isolated primary cell, an insect cell, a yeast (Saccharomyces cerevisiae or Pichia pastoris), a plant cell, a microorganism, or a bacterium (such as E. coli).

The expressions “naturally occurring” or “wild-type” refer to material in the form as it occurs in nature. For example, a naturally occurring or wild-type polypeptide or polynucleotide sequence is a sequence present in an organism that is isolated from a source in nature and which has not been intentionally modified by human manipulation. The expressions “Recombinant”, “engineered” or “non-naturally occurring”: it does not appear in nature, it is an artificial construct, e.g., a cell, nucleic acid, or polypeptide, refers to a material that either has been modified in a manner that would not otherwise be found in nature, or is identical thereto but produced or derived from synthetic materials and/or by manipulation using recombinant techniques.

The expression “reference sequence” refers to a defined sequence to which another sequence is compared. In one implementation, the reference sequence is SEQ ID NO: 2 and preferably SEQ ID NO: 4.

The expression “reference enzyme” is a known enzyme, such as the TACA enzyme or the SspCA enzyme. The activity of the enzyme of the present description is compared to the activity of a reference enzyme.

The expression “coding sequence” refers to the nucleic acid sequence(s) that would yield the amino acid sequence of a given protein/peptide/polypeptide.

The expressions “amino acid”, “residue”, “amino acid residue” refer to the specific monomer at a sequence position of a polypeptide (e.g., 8C indicates that the “amino acid” or “residue” at position 8 of a given sequence is a cysteine (C). The amino acid may be alanine (3 letter code: ala or one letter code: A), arginine (arg or R), asparagine (asn or N), aspartic acid (asp or D), cysteine (cys or C), glutamine (gin or Q), glutamic acid (glu or E), glycine (gly or G), histidine (his or H), Isoleucine (ile or I), leucine (leu or L), lysine (lys or K), methionine (met or M), phenylalanine (phe or F), proline (pro or P), serine (ser or S), threonine (thr or T), tryptophan (trp or W), tyrosine (tyr or Y), valine (val or V). Ter indicates a termination/stop codon.

The expression “amino acid difference” refers to an amino acid at a given position in a protein sequence that is different from the one in the reference sequence. It refers to a change in the amino acid residue at a position of a polypeptide sequence relative to the amino acid residue at a corresponding position in a reference sequence. The positions of amino acid differences generally are referred to herein as “Xn,” where n refers to the corresponding position in the reference sequence upon which the residue difference is based. For example, a “residue difference at position X8 as compared to SEQ ID NO: 7 refers to a change of the amino acid residue at the polypeptide position corresponding to position 8 of SEQ ID NO: 7. Thus, if the reference polypeptide of SEQ ID NO: 7 has a serine at position 8, then a “residue difference at position X8 as compared to SEQ ID NO: 7” an amino acid substitution of any residue other than glycine at the position of the polypeptide corresponding to position 8 of SEQ ID NO: 7. In most instances herein, the specific amino acid residue difference at a position is indicated as “XnY” where “Xn” specifies the corresponding position as described herein, and “Y” is the single letter identifier of the amino acid found in the engineered polypeptide (i.e., the different residue than in the reference polypeptide). In some instances, the present disclosure also provides specific amino acid differences denoted by the conventional notation “AnB”, where A is the single letter identifier of the residue in the reference sequence, “n” is the number of the residue position in the reference sequence, and B is the single letter identifier of the residue substitution in the sequence of the engineered polypeptide. For example, “S8C” would refer to the substitution of the amino acid residue, serine (S) at position 8 of reference sequence with the amino acid cysteine (C). In some instances, a polypeptide of the present disclosure can include one or more amino acid residue differences relative to a reference sequence, which is indicated by a list of the specified positions where changes are made relative to the reference sequence. The present disclosure includes engineered polypeptide sequences comprising one or more amino acid differences that include either/or both conservative and non-conservative amino acid substitutions.

The term “non-conservative substitution” refers to an amino acid, at a given position in a protein sequence that is different and not similar from the one in the reference sequence.

The term “deletion” refers to one or several amino acid(s) at a given position in a protein sequence, that is or are absent when compared to the reference sequence.

The term “insertion” refers to one or several amino acid(s) at a given position in a protein sequence, that is or are in excess when compared to the reference sequence.

The term “improved enzyme property” refers to a property that is better in one enzyme when compared to the reference one. It can be an increase in stability toward some denaturing agent, an increase in thermostability, an increase in solvent stability, an increase in pH stability, an increase in enzyme activity, reduced inhibition by products (e.g. bicarbonate and/or carbonate ions), improved stability in presence of the sodium cation, improved stability in presence of the potassium cation, improved solvent solubility, an increase in hydrophilicity, an increase in hydrophobicity or a combination thereof.

The term “stability in presence of” refers to the capacity of the enzyme to remain active over a period of time when in the presence of a denaturing compound. It is usually described as a percentage of remaining activity over time.

The term “thermostability” refers to the capacity of the enzyme to remain active over a period of time when exposed to a given temperature. It is usually described as a percentage of remaining activity over time.

The term “solvent stability” refers to the capacity of the enzyme to remain active over a period of time when exposed to a given solvent (e.g., a CO₂ capture solvent such as 1.45 M KHCO₃/K₂CO₃ pH 10). It is usually described as a percentage of remaining activity over time.

The term “pH stability” refers to the capacity of the enzyme to remain active over a period of time when exposed to a given pH, such as a higher pH. It is usually described as a percentage of remaining activity over time.

The term “increased enzyme activity” refers to the capacity of an enzyme to catalyze more reaction, such as hydration of CO₂ and/or dehydration of the HCO₃ ⁻ ion, per time unit than the reference enzyme in some given conditions, such as higher Temperature, higher pH (improved pH activity profile).

The term “increase hydrophilicity” refers to the property of the enzyme to be more soluble in water based absorption solution.

The term “increase hydrophobicity” refers to the property of the enzyme to be less soluble in water based absorption solution.

By “about”, it is meant that the relevant value (e.g. of temperature, concentration, pH, etc.) can vary within a certain range depending on the margin of error of the method or apparatus used to evaluate such value. For instance, the margin of error of the temperature may range between ±0.5° C. to ±1° C., the margin of error of the pH may be ±0.1 and the margin of error of the concentration may be ±20%.

In some implementations, the TACA or functional derivative thereof can be used in a CO₂ capture operation where the absorption and desorption stage are run within certain temperature conditions to leverage TACA or functional derivative thereof s temperature and solvent stability. For example, the absorption stage can be operated between 40° C. and 60° C. and the desorption stage can be operated between 40° C. and 85° C. The absorption and desorption stages can also be configured such that the TACA or functional derivative thereof flows through each stage and has residence times within each stage that further leverage the TACA or functional derivative thereof s temperature and solvent stability. For example, the residence time in the absorption stage can be 1 minute to 20 minutes and the residence time in the desorption stage can be 1 minute to 10 minutes. In addition, the concentration of the TACA or functional derivative thereof in the solution can be provided such that catalytic activity is promoted for enhanced residual activity in the CO₂ capture process. For example, the TACA or functional derivative thereof can be provided in sufficiently high concentration so as to maintain near 100% residual activity through at least 14 days of operation.

The tests show that the TACA or functional derivative thereof was better than all other tested enzymes between 60 and 98° C. after a certain amount of time. Since TACA or the functional derivative thereof is stable, it may maintain close to 100% residual activity over all temperatures for at least 1 hour. Activity determinations are conducted so there is no over-saturation with enzyme.

As the TACA or functional derivative thereof has been found to have higher residual activity than all of the comparative carbonic anhydrases that were tested, as illustrated in the examples section, the TACA or functional derivative thereof can be used in a CO₂ capture operation with greater efficiency and performance compared to other carbonic anhydrases.

In some implementations, the TACA or functional derivative thereof can be used to “top-up” or replenish carbonic anhydride-based CO₂ capture operations. The TACA or functional derivative thereof “top-up” or replenishing frequency and amount can be provided such that high catalysis is maintained.

In some implementations, the recombinant TACA or functional derivative thereof can have an improved property relative to the same property of the polypeptide of SEQ ID NO: 4, selected from one or more of improved stability and or activity and or solubility in presence of sodium ion; improved stability and or activity and or solubility in presence of potassium ion; improved stability and or activity and or solubility in presence of carbonate ion; improved stability and or activity and or solubility under high pH conditions; improved stability and or activity and or solubility under high temperature conditions and improved pH-activity profile.

In addition, the TACA or functional derivative thereof assessed in tests reported in the present description display enhanced stability compared to other TACAs assessed by James et al., for example. In James et al., a mild HEPES/NaCl buffer was used and the enzyme was exposed to 90° C. for one hour, resulting in complete deactivation. In contrast, the TACA or functional derivative thereof enzymes of the present disclosure and having structural differences compared to the James et al. enzymes gave enhanced results in terms of enzyme stability. For example, as shown in FIG. 6, TACA retained nearly 100% residual activity when exposed for one hour at 98° C. in 1.45 M K₂CO₃ at pH 10.

Various aspects of the present description will be more readily understood by referring to the following examples. These examples are illustrative of the wide range of applicability of the present description and are not intended to limit its scope. Modifications and variations can be made therein without departing from the spirit and scope of the description. Although any methods and materials similar or equivalent to those described herein can be used in the practice for testing of the present description, the preferred methods and materials are described.

The scope of the claims should not be limited by the aspects, scenarios, implementations, examples or embodiments set forth in the examples and the description, but should be given the broadest interpretation consistent with the description as a whole.

The issued patents, published patent applications, and references that are mentioned herein are hereby incorporated by reference. In the case of inconsistencies, the present disclosure will prevail.

EXAMPLES Example 1: Materials, Methods and Producing of TACA Enzymes

TACA enzymes were constructed, expressed, and purified as follows. Using standard recombinant methods, the first 20 amino acids of the wild-type TACA enzyme (see FIG. 1; SEQ ID NO: 2), corresponding to its N-terminal signal sequence, were removed and replaced with a single methionine residue, giving a truncated TACA enzyme having the amino acid sequence of SEQ ID NO: 4. It was subsequently found that higher expression levels of the truncated TACA enzyme in a bacterial expression host could be obtained by further replacing the first six amino acid residues (“MGGGAH”) of the enzyme of SEQ ID NO: 4 with the residues “MEHE”, giving the TACA enzyme set forth in SEQ ID NOs: 6 and 7. For greater clarity, SEQ ID NOs: 6 and 7 refer to the same protein and may be used interchangeably.

Following codon-optimization of the TACA coding sequences, they were cloned into expression vectors which employed either a T5 or T7 promoter and expressed in an E. coli BL21-derived strain. TACA enzymes were purified by heat purification and other standard techniques.

Following its purification, the TACA enzyme having the amino acid sequence of SEQ ID NO: 4 (hereafter referred to as “wtTACA”) was characterized in a CO₂ capture column and by a pH indicator-based technique. The CO₂ capture column consists in contacting a gas containing 14% v/v CO₂ and a CO₂-capture solvent consisting of 1.45 M KHCO₃/K₂CO₃ pH 10 at 25° C. When present, the enzyme is dissolved in the solvent at a concentration of 0.2 g/L. The solvent flows inside a 50 cm height packed column from top to the bottom. The CO₂-containing gas flows counter currently inside the same column. The liquid to gas flowrate ratio is adjusted to 50 g/g. A gas analyzer measures the CO₂ concentration in the gas at the inlet and outlet of the column. In these conditions, a 90% CO₂ removal efficiency was obtained confirming the high activity level of TACA.

The pH indicator-based technique was performed to compare the stability and activity of TACA with those of other carbonic anhydrases. More specifically, the TACA of SEQ ID NO: 4 was compared with the following other carbonic anhydrases:

-   -   (i) Carbonic anhydrase from Sulfurihydrogenibium sp. referred as         “SspCA” (SEQ ID NO: 8) and described in patent application         WO2014066999 A1, which shares 49% amino acid sequence identity         with SEQ ID NO: 4 by performing a protein BLAST; and     -   (ii) A thermostable variant of SspCA referred to as “6M1” (SEQ         ID NO: 9), described in patent application WO2014066999 A1 (see         SEQ ID NO: 196 therein), which shares 50% amino acid sequence         identity with SEQ ID NO: 4 by performing a protein BLAST.

Using the pH-indicator-based technique, the three tested carbonic anhydrases (i.e., TACA, SspCA, and 6M1; SEQ ID NOs: 4, 8, and 9, respectively) returned about the same activity level. To assess their respective stabilities, their residual activity was evaluated after 1 h exposure at 98° C. in 1.45 M K₂CO₃ pH 10. While the TACA of SEQ ID NO: 4 retained nearly 100% residual activity, SspCA and 6M1 retain only about 5% and 30% residual activities, respectively.

Example 2: Performance of TACA in a Packed Column Absorption Unit

An experiment was conducted in an absorption packed column. The absorption solution is an aqueous solution of potassium carbonate 1.45 M at pH 10. This absorption solution is contacted counter-currently with a gas phase with a CO₂ concentration of 130,000 ppm. Liquid flow rate was 500 g/min and gas flow rate was 10 g/min corresponding to L/G of 50 g/g. Gas and absorption solution were at room temperature. The column has a 7.5 cm diameter and a 50 cm height. Packing material is polymeric Raschig™ rings 6 mm. The TACA concentration was 0.2 g/L. The results showed that CO₂ transfer rate of CO₂ removal rate increased from 4.7 mmol/sec for the solution to 40 mmol/sec when adding the enzyme to the absorption solution. TACA (SEQ ID NO: 4) increased the CO₂ removal rate by 8.5 fold under these conditions.

Example 3: Stability of TACA Compared to that of SspCA and 6M1

The stability of TACA, SspCA and 6M1 (SEQ ID NOs: 4 & 6, 8, and 9, respectively) enzymes was compared. The stability was evaluated by exposing the enzymes to an absorption solution including 1.45 M KHCO₃/K₂CO₃ (2.9 M K⁺) pH 10 and 20% w/v MDEA alpha=0.1 at various temperatures for different exposure times. As shown in FIGS. 2 to 9, TACA (both SEQ ID NOs: 4 and 6) exhibited the highest stability in all the tested conditions, as compared to the SspCA and 6M1 enzymes.

As shown in FIG. 3, in 1.45 M KHCO₃/K₂CO₃ (2.9M K⁺) pH 10, TACA retained all of its activity after one week incubation at 60° C., while the SspCA and 6M1 enzymes lost more than 60% of their initial activity. Furthermore, TACA showed 50% residual activity after 60 hours incubation at 75° C., while other enzyme returned 10% or less residual activity levels (FIG. 4). TACA also retained the most activity at higher temperatures, such as at 85° C. (FIG. 5) and at 98° C. (FIG. 6).

As shown in FIG. 8, TACA retained 100% of its initial activity after 28 days incubation at 60° C. in 20% MDEA alpha=0.1. During the same time, SspCA was completely inactivated while 6M1 still exhibited some activity.

Example 4: Stability of TACA Compared to that SspCA and 6M1 in the Context of Thermal Cycling in 1.45 M KHCO₃/K₂CO₃ (2.9M K⁺) pH 10

In industrial application, enzymes will have to deal with temperature fluctuations. To test the enzyme stability in this context, a thermal cycling test was conducted on TACA. The enzyme was subjected to temperature fluctuations occurring between 30° C. and 75° C. FIG. 9 shows temperature profile occurring for each cycle which lasts about 8 minutes. This cycle was repeated 180 times per day for 28 days, giving a total of 5040 cycles. Under these conditions, TACA retained about 50-100% residual activity level after 7-14 days. About 25-50% activity level was recorded after 28 days.

Example 5: Comparison of Amino Acid Sequences Between Carbonic Anhydrase Obtained from Thermovibrio ammonificans and the Most Similar Protein in GenBank

The Table below shows sequence similarities between TACA and the most similar proteins in GenBank, including a carbonic anhydrase from Persephonella marina SspCA and the variant 6M1 from said SspCA, which were identified by performing a protein BLAST against known sequences in GenBank.

As shown in the Table below, the most similar carbonic anhydrases to the TACA carbonic anhydrase obtained from Thermovibrio ammonificans was found in P. marina with 66% identity. SspCA, not shown in the Table below, was ranked as the 375^(th) most similar protein.

Genbank accession Query number Description cover Identity WP_013538320.1 carbonic anhydrase  98% 100% [Thermovibrio ammonificans] WP_015898908.1 carbonic anhydrase  98%  66% [Persephonella marina] WP_029522463.1 carbonic anhydrase  98%  63% [Persephonella sp. KM09-Lau-8] WP_029521561.1 carbonic anhydrase  98%  61% [Persephonella sp. IF05-L8] WP_007474387.1 carbonic anhydrase  98%  59% [Caminibacter mediatlanticus] WP_028579713.1 hypothetical protein  98%  52% [Desulfobulbus japonicus] WP_019445033.1 carbonic anhydrase  98%  53% [Aeromonas sp. 159] WP_007040788.1 carbonic anhydrase  98%  52% [Thiorhodococcus drewsii] WP_005354260.1 carbonic anhydrase  98%  53% [Aeromonas veronii] WP_005362587.1 carbonic anhydrase  98%  53% [Aeromonas veronii] WP_005348316.1 carbonic anhydrase  98%  53% [Aeromonas veronii] WP_007766615.1 carbonic anhydrase 100%  49% [Cronobacter turicensis] WP_012459296.1 carbonic anhydrase  97%  49% [Sulfurihydrogenibium sp. YO3AOP1 (SspCA)] not applicable SspCA variant “6M1”  97%  50% (SEQ ID NO: 9)

Example 6: TACA's Stability Improvement in Carbonate-Based Buffer

Recombinant (or engineered) carbonic anhydrase (CA) polypeptides having improved properties relative to wild-type TACA (SEQ ID NO: 4) were generated. The latter CAs are hereafter referred to as improved variants. The improved variants were generated using directed evolution techniques that are well known by those skilled in the art.

The improved properties included one or more of: improved thermostability, improved activity (hydration of CO₂ and/or dehydration of the HCO₃ ⁻ ion), improved high pH stability (e.g. pH 7 to 12), improved pH activity profile, reduced inhibition by products (e.g., bicarbonate and/or carbonate ions), improved stability in presence of the sodium cation, improved stability in presence of the potassium cation, improved solvent solubility, reduced inhibition by gas contaminants, or any combination thereof.

The improved variants comprise at least one or more amino acid substitutions in their amino acid sequence relative to that of wild-type TACA (SEQ ID NO: 4) or the truncated TACA N-terminal derivative of SEQ ID NO: 6 or 7, that results in CA exhibiting improved properties. An improved variant can have in its amino acid sequence 1 or more substitutions, 2 or more substitutions, 3 or more substitutions, 4 or more substitutions, 5 or more substitutions, 6 or more substitutions, 7 or more substitutions, 8 or more substitutions, 9 or more substitutions, 10 or more substitutions. The improved variant may additionally comprise neutral mutations. The improved variant can be substantially identical to TACA. By “substantially identical”, it is meant that the sequence of the present description has an amino acid sequence which is at least about 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 99.5% identical to SEQ ID NO: 4, 6, 7 or 10. The mutations comprise but are not limited to any mutations at positions listed in Table 3, or any functional derivative thereof. The mutation can be conservative or non-conservative. Non-limiting examples of conservative mutations are given in Table 1. Conservative mutations are known to usually provide similar effect to protein structure and function. The functional derivative can comprise substitution(s), insertion(s) and/or deletion(s), or combination thereof. The variant can be free or immobilized.

TABLE 1 Possible conservative mutations Class Amino acid Conservative mutation class Non-polar A, V, L, I Non-polar Other non-polar Other non-polar G, M Non-polar Aromatic H, F, Y, W Aromatic Polar Q, N, S, T Polar > acidic, basic Acidic D, E Acidic > polar Basic K, R Basic > polar Other C, P None

The functional derivative can have any substitution at surface-exposed residues. It is known by those skilled in the art that most neutral substitutions, i.e. mutations that retain biological and biophysical properties of a given protein, are found at these positions. Mutations tend also to be found at residues not involved in the function of the protein and away from the active site region. Table 2 describes the location and features of every TACA residue in its 3D-structure (PDB ID 4C3T).

TABLE 2 Location and features of TACA residues based on their 3D-structure Position Solvent exposure Other feature X1 Exposed X2 Exposed X3 Exposed X4 Exposed X5 Exposed X6 Partially exposed X7 Exposed X8 Exposed X9 Exposed X10 Exposed X11 Exposed X12 Partially exposed X13 Exposed X14 Exposed X15 Exposed X16 Buried X17 Partially exposed X18 Exposed X19 Exposed X20 Exposed X21 Exposed X22 Exposed X23 Exposed X24 Exposed Dimer interface X25 Exposed Dimer interface X26 Partially exposed Intramolecular disulfide bridge X27 Exposed X28 Exposed Dimer interface X29 Exposed X30 Exposed Dimer interface X31 Buried Dimer interface X32 Exposed X33 Buried X34 Buried X35 Partially exposed X36 Exposed X37 Partially exposed X38 Exposed X39 Exposed X40 Exposed X41 Exposed X42 Exposed X43 Exposed Dimer interface X44 Exposed X45 Partially exposed Dimer & tetramer interface X46 Exposed Intermolecular disulfide bridge, Tetramer interface X47 Partially exposed X48 Exposed X49 Exposed X50 Partially exposed X51 Exposed X52 Exposed X53 Exposed X54 Exposed X55 Exposed X56 Exposed X57 Exposed X58 Exposed X59 Exposed X60 Exposed X61 Buried X62 Exposed X63 Partially exposed X64 Exposed X65 Exposed X66 Exposed Proton shuttle X67 Exposed X68 Buried X69 Exposed X70 Buried X71 Exposed X72 Partially exposed X73 Exposed X74 Exposed X75 Exposed X76 Partially exposed X77 Exposed X78 Buried X79 Exposed X80 Partially exposed X81 Exposed Dimer interface X82 Exposed X83 Exposed X84 Exposed X85 Buried X86 Exposed X87 Buried X88 Exposed X89 Exposed X90 Buried X91 Buried Metal coordinating X92 Buried X93 Buried Metal coordinating X94 Partially exposed X95 Exposed X96 Buried X97 Buried X98 Buried X99 Exposed X100 Exposed X101 Exposed X102 Exposed X103 Exposed X104 Exposed X105 Exposed X106 Exposed X107 Buried X108 Buried X109 Buried X110 Buried Metal coordinating X111 Buried X112 Exposed Active site pocket X113 Buried X114 Exposed X115 Exposed X116 Exposed X117 Exposed X118 Exposed X119 Exposed X120 Exposed X121 Buried X122 Partially exposed Active site pocket X123 Buried X124 Buried X125 Buried X126 Buried X127 Buried X128 Exposed X129 Exposed X130 Exposed X131 Exposed X132 Exposed X133 Exposed X134 Exposed X135 Exposed X136 Buried X137 Exposed X138 Exposed X139 Buried X140 Partially exposed X141 Exposed X142 Exposed X143 Exposed X144 Exposed X145 Exposed X146 Exposed X147 Exposed X148 Exposed X149 Exposed X150 Exposed X151 Exposed X152 Exposed X153 Partially exposed X154 Exposed X155 Exposed X156 Exposed X157 Buried X158 Exposed X159 Exposed X160 Exposed X161 Exposed X162 Buried X163 Partially exposed X164 Partially exposed X165 Exposed X166 Exposed X167 Exposed X168 Exposed X169 Exposed X170 Exposed X171 Buried Dimer interface X172 Buried X173 Buried Dimer interface X174 Buried X175 Buried X176 Exposed Active site pocket X177 Exposed Active site pocket X178 Exposed Active site pocket X179 Exposed Active site pocket X180 Exposed Active site pocket X181 Buried Intermolecular disulfide bridge X182 Exposed X183 Exposed Dimer interface X184 Exposed Dimer interface X185 Partially exposed X186 Partially exposed Dimer interface X187 Buried X188 Buried X189 Buried X190 Buried X191 Exposed X192 Exposed X193 Exposed X194 Exposed X195 Exposed X196 Buried X197 Exposed X198 Exposed X199 Exposed X200 Partially exposed X201 Partially exposed X202 Exposed X203 Exposed X204 Buried X205 Exposed X206 Exposed X207 Exposed X208 Exposed X209 Exposed X210 Exposed X211 Exposed X212 Buried X213 Exposed X214 Partially exposed X215 Exposed X216 Exposed X217 Exposed X218 Exposed X219 Exposed Dimer interface X220 Exposed Dimer interface X221 Buried Dimer interface X222 Buried Dimer interface X223 Partially exposed Dimer interface X224 Buried X225 Buried Dimer interface X226 Partially exposed Tetramer interface

A number of TACA variants displaying improved stability, as compared to the TACA enzyme of SEQ ID NO: 7, were identified using directed evolution techniques. These directed evolution techniques involved exposing the variants to conditions suitable for CO₂-capture (1.45 M K₂CO₃ pH 10 at temperatures ≥70° C.), and measuring their activity. Variants that were made and tested include those shown in Table 3.

TABLE 3 TACA variants tested for enhanced stability in 1.45M K₂CO₃ pH 10 at a temperature ≥ 70° C. Amino acid substitution (residue numbering according to SEQ ID NO: 7) Temperature 3-letter code 1-letter code 90° C. 85° C. 70° C. Glu2Gln E2Q ★ − − Glu2Val E2V ★★ − − Trp5Cys W5C ★★★ ★★★ ★★★★ Trp5Arg W5R ★★★ ★★★ NA Tyr7Cys Y7C ★★★ ★ NA Tyr7His Y7H ★★★ ★★ NA Tyr7Phe Y7F ★★★ ★★ NA Ser8Cys S8C ★★★ ★★ ★★★★★ Gly9Cys G9C ★★ ★★ − Gly9Pro G9P ★★ ★★ − Gly9Asn G9N NA NA NA Gly12Asp G12D ★ ★ − Gly12Arg G12R ★★ ★★ ★ Gly12Val G12V ★★ ★ ★ Pro13Gln P13Q ★★ ★ − Trp16Arg W16R ★★ ★★ NA Trp16Cys W16C ★★ − NA Trp16Gly W16G ★★ ★ NA Gly17Cys G17C ★★★ ★★★ NA Asp18Val D18V ★★★ ★★ NA Pro21Leu P21L ★ ★ ★★ Pro21Lys P21K − − Glu22Lys E22K − ★ − Glu22Pro E22P ★★★★ ★★★★ ★ Glu22Leu E22L ★★ ★★★ − Asn31Cys N31C − − − Ser51Ile S51I − − − Va152Ile V52I − ★ ★★★ Gly73Cys G73C ★ ★ ★★ Gly76Asp G76D − − − Tyr77Asp Y77D ★★ ★ − Gly82Ser G82S − − − Asn101Gln N101Q − − − Lys103Met K103M − − NA Asp115Asn D115N − − − Lys116Asn K116N − − ★ Asn117Gln N117Q − − − Val125Ala V125A ★★ ★ − Phe126Leu F126L − − ★ Lys131Ile K131I ★★ ★★ − Lys138Asn K138N ★ − − Arg141His R141H − − − Arg151Cys R151C − − − Ser173Cys S173C − − − Cys181Tyr C181Y − − − Phe190Cys F190C − − ★★ Gly12Asp, Pro21Gln G12D, P21Q NA NA NA Gly12Asp, K27Thr G12D, K27T NA NA NA Gly12Asp, Val142Ile G12D, V142I NA NA NA Trp16Cys, Arg167Ser W16C, R167S NA NA NA Trp16Arg, Lys206Thr W16R, K206T NA NA NA Trp16Gly, Val50Met W16G, V50M NA NA NA Leu19Ser, Lys138Arg L19S, K138R NA NA NA Leu19Ser, Asp158Gly L19S, D158G NA NA NA Lys150Met, Pro193Ser K150M, P193S − ★ Phe190Met, Pro193Ser F190M, P193S ★ ★ NA Arg141His, Arg151Cys R141H, R151C − ★ − Ser8Cys, Arg141His S8C, R141H ★★ ★ − Ser8Cys, Arg151Cys S8C, R151C ★★★ ★★ − Ser8Cys, Arg141His, S8C, R141H, R151C ★ − ★★ Arg151Cys Gly9Pro, Pro21Asp G9P, P21D NA NA NA Gly9Pro, Pro21Asn G9P, P21N NA NA NA Gly9Pro, Pro21Gln G9P, P21Q NA NA NA Gly9Pro, Pro21Val G9P, P21V NA NA NA Gly9Pro, Pro21Gly G9P, P21G NA NA NA Gly9Pro, Glu22Pro G9P, E22P ★★★★ ★★★★ − Ser8Arg, Gly9Pro, Glu22Pro S8R/G9P/E22P ★★★★★ ★★★★★ ★★★★ Ter227Tyr, Gly228, Ter* Ter227Y, G228, Ter* ★ ★★ ★★★ Ser8Arg S8R ★ ★ NA Arg156Glu R156E ★★ ★★ ★★★ Ser8Arg, Gly9Pro, Glu22Pro, S8R/G9P/E22P/R156E ★★★★ ★★★★ ★★★ Arg156Glu *“Ter” is the abbreviation for the Stop codon. The stop codon in SEQ ID NO: 7 was replaced with the insertion of Tyr-Gly in the variant “Ter227Y, G228, Ter” − Half-life about that of SEQ ID NO: 7 or less ★ Half-life between 100% to 125% that of SEQ ID NO: 7 ★★ Half-life between 125% to 150% that of SEQ ID NO: 7 ★★★ Half-life between 150% to 200% that of SEQ ID NO: 7 ★★★★ Half-life between 200% to 300% that of SEQ ID NO: 7 ★★★★★ Half-life over 300% that of SEQ ID NO: 7

Strikingly, the above data clearly show that most of the beneficial mutations in the enzymatic conditions tested are found in the first 22 residues of TACA. Moreover, multiple mutations were identified at nine amino acid positions (i.e., X2, X5, X7, X8, X9, X12, X16, X21, and X22). Taken together, these results suggest that this region of the wild-type TACA enzyme is not optimal for CO₂ capture, and may be engineered to obtain an enzyme having improved properties in a CO₂ capture solvent. These results also show that there are multiple routes possible, for someone skilled in the art of protein engineering, to improve this particular region.

Example 6b: Stability of the TACA S8R/G9P/E22P Variant in Different CO₂ Capture Solvents

The impact of mutations in various CO₂ capture solvent was assessed. To do so, stability of TACA (SEQ ID NO: 7) was compared to that of a TACA variant containing three mutations (i.e., S8R/G9P/E22P). Both enzymes were incubated at 85° C. in these CO₂ capture solvents:

-   -   1.45 M K₂CO₃ pH 10.0     -   0.3 M Na₂CO₃ pH 10.0     -   2 M MDEA     -   2.4 M potassium N,N-dimethylglycine (pH 10.3)

For all tested solvents, a 2 g/L enzyme concentration was used. Residual activity level was determined over time.

As shown in FIG. 14, the TACA S8R/G9P/E22P variant showed increased stability in comparison to the TACA of SEQ ID NO: 7 in all of the tested solvents. These results confirm that, despite the fact that these mutants were initially selected for their higher stability in 1.45 M K₂CO₃, they can confer a beneficial impact in other solvents. That being said, the extent of the stability improvement provided by the variants, may vary from one solvent to another. For this particular TACA S8R/G9P/E22P variant, the stability increase was greater in for example 2 M MDEA than in 2.4 M potassium N,N-dimethylglycine.

Example 7: Stability of the TACA Under Temperature Cycling Conditions

To confirm the potential of TACA (or functional derivatives thereof) for use in CO₂ capture operations, its stability was evaluated under temperature cycling conditions to mimic the process conditions to which it would be exposed. 1.2 L of a 1.45 M K₂CO₃ solution at a CO₂ loading of 0.63 (pH 10), containing 2 g/L of TACA enzyme (SEQ ID NO: 6), exposed to a 40° C. was continuously pumped through a water bath at a temperature of 77° C. where its temperature was increased for 4 minutes. Then the solution was pumped back to the reservoir at 40° C. A temperature of 40° C. is typical of conditions in an absorption unit and higher temperatures are representative of temperature to be encountered in a desorption unit. The solution was exposed to these temperature cycling conditions on 24h per day and 7d per week basis. At specific exposure times, samples of the solution were withdrawn for activity measurement. CO₂ hydration activity of TACA or functional derivative thereof was measured at 25° C. in a 1.45 M K₂CO₃ pH 10 solution, TACA or derivative thereof concentration for the assay was 0.2 g/L.

Residual activity data for TACA or derivative thereof are shown in FIG. 15. The results show that this enzyme keeps at least 80% of its initial activity for at least 20 days. In the context of an industrial use of this enzyme in a CO₂ capture unit, this clearly demonstrates that the enzyme is robust towards industrially relevant operation conditions characterized by salt concentration higher than 0.5 M and alkaline pH.

These tests show that TACA or a functional derivative thereof has remarkable stability even in practical process conditions, which are relatively harsh when compared to standard laboratory conditions.

Example 8: Cyclic Process Performance

The industrial relevance of TACA (SEQ ID NO: 6 or 7) was demonstrated in a 1 ton per day CO₂ capture pilot unit located at the University of North Dakota's Energy & Environmental Research Center (EERC). The CO₂ capture unit included a packed column absorber and a packed column stripper/desorber. The TACA enzyme was used in combination with a 1.45 M K₂CO₃ solution to capture CO₂ from a gas effluent. Two types of gas effluents were tested: one from natural gas combustion and a second from coal combustion. CO₂ concentration in the flue gas from the natural gas combustion had a concentration of 10% (v/v) and the one coming from coal combustion had a concentration of 14% (v/v). In addition to CO₂, flue gases included contaminants such as CO, NOx, and/or SOx coming from coal combustion.

The packed column absorber was operated at 30° C. The absorption solution containing potassium carbonate and TACA was fed at the top of the absorber. As the solution counter currently contacted the flue gas, it absorbed CO₂ so the pH of the solution went from 10.2 to 9.1. In order to strip the CO₂ out of the absorption solution, the CO₂ loaded solution was sent to a stripper where it was heated using a heating medium at a temperature of 85° C. The CO₂ was released from the solution as a concentrated CO₂ stream. The absorption solution, now a CO₂ lean solution, was sent back to the absorber.

TACA enzyme concentrations were varied from 0.2 to 2 g/L. Results are shown in FIG. 16 and indicate that a small enzyme concentration of 2 grams per liter is sufficient to cause an increase in CO₂ capture performance by near five-fold under tested conditions. The enzyme was used in the pilot unit for 7 days, 24 h/day, without any activity decrease, even when contacted with gas contaminants such as NOx, CO, and/or SOx, confirming the industrial relevance of TACA for CO₂ capture operations. 

What is claimed is:
 1. A recombinant carbonic anhydrase polypeptide having carbonic anhydrase activity comprising an amino acid sequence having at least 90% identity to SEQ ID NO: 7, and one or more differences as compared to SEQ ID NO: 7 at residue positions selected from 2, 5, 7, 8, 9, 12, 13, 16, 17, 18, 19, 21, 22, 52, 73, 77, 116, 125, 126, 131, 138, 156, 190, 193, and
 206. 2. The recombinant polypeptide of claim 1, wherein said amino acid difference is at a position corresponding to position 2, 5, 7, 8, 9, 12, 13, 16, 17, 18, 19, 21, 22, or any combination thereof, of SEQ ID NO:
 7. 3. The recombinant carbonic anhydrase polypeptide of claim 1, comprising one or more amino acid differences as compared to SEQ ID NO: 7 selected from: 2Q or 2V; 5C or 5R; 7C, 7F, or 7H; 8C or 8R; 9C, 9P or 9N; 12D, 12R or 12V; G13Q; 16C, 16G, or 16R; 17C; 18V; 19S; 21L, 21D, 21N, 21Q, 21V, or 21G; 22K, 22L, 22P, or 22N; 52I; 73C; 77D; 116N; 125A; 126L; 131I; 138N or 138R; 156E; 190C or 190M; 193S; and 206T.
 4. The recombinant carbonic anhydrase polypeptide of claim 1 comprising two or more amino acid differences as compared to SEQ ID NO: 7 which are: 12D and 21Q; 12D and 27T; 12D and 142I; 16C and 167S; 16R and 206T; 16G and 50M; 19S and 138R; 19S and 158G; 150M and 193S; 190M and 193S; 141H and 151C; 8C and 141H; 8C and 151C; 8C, 141H, and 151C; 9P and 21D; 9P and 21N; 9P and 21Q; 9P and 21V; 9P and 21G; 9P and 22P; 8R, 9P, and 22P; 8R, 9P, 22P, and 156E.
 5. The recombinant carbonic anhydrase polypeptide of claim 1, further comprising 227Y and 228G.
 6. The recombinant carbonic anhydrase polypeptide of claim 1, having at least 90% identity to SEQ ID NO:7.
 7. The recombinant carbonic anhydrase polypeptide of claim 1, having at least 95% identity to SEQ ID NO:7.
 8. The recombinant carbonic anhydrase polypeptide of claim 1, comprising one or more amino acid differences as compared to SEQ ID NO: 7 selected from: (a) 8C or 8R; (b) 9C, 9P or 9N; (c) 22K, 22L, 22P, or 22N; or (d) any combination of (a) to (c), wherein said recombinant carbonic anhydrase polypeptide exhibits improved stability in a CO2 capture solvent comprising carbonate ions as compared to the polypeptide of SEQ ID NO:7.
 9. A recombinant carbonic anhydrase polypeptide having carbonic anhydrase activity comprising: (a) an amino acid sequence having at least 90% identity to SEQ ID NO:7; and (b) one or more differences as compared to SEQ ID NO: 7 at residue positions selected from 2, 5, 7, 8, 9, 12, 13, 16, 17, 18, 19, 21, 22, 52, 73, 77, 116, 125, 126, 131, 138, 156, 190, 193, and 206, wherein said recombinant carbonic anhydrase polypeptide has improved stability relative to the carbonic anhydrase of SEQ ID NO: 7, following 1 hour of exposure in 1.45 M K₂CO₃ pH 10 at 70° C., 85° C. or 90° C.
 10. The recombinant polypeptide of claim 9, wherein said amino acid difference is at a position corresponding to position 2, 5, 7, 8, 9, 12, 13, 16, 17, 18, 19, 21, 22, or any combination thereof, of SEQ ID NO:
 7. 11. The recombinant carbonic anhydrase polypeptide of claim 9, comprising one or more amino acid differences as compared to SEQ ID NO: 7 selected from: 2Q or 2V; 5C or 5R; 7C, 7F, or 7H; 8C or 8R; 9C, 9P or 9N; 12D, 12R or 12V; G13Q; 16C, 16G, or 16R; 17C; 18V; 19S; 21L, 21D, 21N, 21Q, 21V, or 21G; 22K, 22L, 22P, or 22N; 52I; 73C; 77D; 116N; 125A; 126L; 131I; 138N or 138R; 156E; 190C or 190M; 193S; and 206T.
 12. The recombinant carbonic anhydrase polypeptide of claim 9 comprising two or more amino acid differences as compared to SEQ ID NO: 7 which are: 12D and 21Q; 12D and 27T; 12D and 142I; 16C and 167S; 16R and 206T; 16G and 50M; 19S and 138R; 19S and 158G; 150M and 193S; 190M and 193S; 141H and 151C; 8C and 141H; 8C and 151C; 8C, 141H, and 151C; 9P and 21D; 9P and P21N; 9P and 21Q; 9P and 21V; 9P and 21G; 9P and 22P; 8R, 9P, and 22P; 8R, 9P, 22P, and 156E.
 13. The recombinant carbonic anhydrase polypeptide of claim 9, further comprising 227Y and 228G.
 14. The recombinant carbonic anhydrase polypeptide of claim 9, having at least 85% identity to SEQ ID NO:
 7. 15. The recombinant carbonic anhydrase polypeptide of claim 9, having at least 90% identity to SEQ ID NO:
 7. 16. The recombinant carbonic anhydrase polypeptide of claim 9, having at least 95% identity to SEQ ID NO:
 7. 17. The recombinant carbonic anhydrase polypeptide of claim 9, comprising one or more amino acid differences as compared to SEQ ID NO: 7 selected from: (a) 8C or 8R; (b) 9C, 9P or 9N; (c) 22K, 22L, 22P, or 22N; or (d) any combination of (a) to (c).
 18. A recombinant variant of a Thermovibrio ammonificans carbonic anhydrase polypeptide, said Thermovibrio ammonificans carbonic anhydrase polypeptide comprising an amino acid sequence that is at least 90% identical to the sequence of SEQ ID NO: 7, the variant having carbonic anhydrase activity and comprising one or more amino add differences as compared to SEQ ID NO: 7 at residue positions selected from 2, 5, 7, 8, 9, 12, 13, 16, 17, 18, 19, 21, 22, 52, 73, 77, 116, 125, 126, 131, 138, 156, 190, 193, and 206, wherein said variant has improved stability relative to the carbonic anhydrase of SEQ ID NO: 7, following 1 hour of exposure in 1.45 M K₂CO₃ pH 10 at 70° C., 85° C. or 90° C.
 19. The recombinant carbonic anhydrase polypeptide of claim 18, comprising one or more amino acid differences as compared to SEQ ID NO: 7 selected from: 2Q or 2V; 5C or 5R; 7C, 7F, or 7H; 8C or 8R; 9C, 9P or 9N; 12D, 12R or 12V; G13Q; 16C, 16G, or 16R; 17C; 18V; 19S; 21L, 21D, 21N, 21Q, 21V, or 21G; 27T; 50M; 52I; 73C; 77D; 116N; 125A; 126L; 131I; 138N or 138R; 141H; 142I; 150M; 151C; 156E; 158G; 167S; 173C; 181Y; 190C or 190M; 193S; and 206T.
 20. The recombinant carbonic anhydrase polypeptide of claim 18, comprising two or more amino acid differences as compared to SEQ ID NO: 7 which are: 12D and 21Q; 12D and 27T; 12D and 142I; 16C and 167S; 16R and 206T; 16G and 50M; 19S and 138R; 19S and 158G; 150M and 193S; 190M and 193S; 141H and 151C; 8C and 141H; 8C and 151C; 8C, 141H, and 151C; 9P and 21D; 9P and P21N; 9P and 21Q; 9P and 21V; 9P and 21G; 9P and 22P; 8R, 9P, and 22P; 8R, 9P, 22P, and 156E. 